Datums in Texas NGS: Welcome to Geodesy

Geodesy is the science of measuring and monitoring the size and shape of the Earth and the location of points on its surface. NOAA's National Geodetic Survey (NGS) is responsible for the development and maintenance of a national geodetic data system that is used for navigation, communication systems, and mapping and charting. ln this subject, you will find three sections devoted to learning about geodesy: an online tutorial, an educational roadmap to resources, and formal lesson plans.

The Geodesy Tutorial is an overview of the history, essential elements, and modern methods of geodesy. The tutorial is content rich and easy to understand. lt is made up of 10 chapters or pages (plus a reference page) that can be read in sequence by clicking on the arrows at the top or bottom of each chapter page. The tutorial includes many illustrations and interactive graphics to visually enhance the text.

The Roadmap to Resources complements the information in the tutorial. The roadmap directs you to specific geodetic data offered by NOS and NOAA. The Lesson Plans integrate information presented in the tutorial with data offerings from the roadmap. These lesson plans have been developed for students in grades 9-12 and focus on the importance of geodesy and its practical application, including what a datum is, how a datum of reference points may be used to describe a location, and how geodesy is used to measure movement in the Earth's crust from seismic activity.

Members of a 1922 geodetic suruey expedition. Until recent advances in satellite technology, namely the creation of the Global Positioning Sysfem (GPS), geodetic surveying was an arduous fask besf suited to individuals with strong constitutions, and a sense of adventure.

Geodesy is the science of measuring and monitoring the size and shape of the Earth. Geodesists basically assign addresses to points all over the Earth. lf you were to stick pins in a model of the Earth and then give each of those pins an address, then you would be doing what a geodesist does. By looking at the height, angles, and distances between these locations, geodesists create a spatial reference system that everyone can use. Building roads and bridges, conducting land surveys, and making maps are some of the important activities that depend on a spatial reference system. For example, if you build a

Compiled by Jib Ahmad, RPLS, CFM - September 2014 Page I of 25 Datums in Texas bridge, you need to know where to start on both sides of the river. lf you don't, your bridge may not meet in the middle.

As positioning and navigation have become fundamental to the functions of society, geodesy has become increasingly important. Geodesy helps the transportation industry ensure safety and reliability, while reducing costs. Without geodesy, planes might land next to -- rather than at -- airports, and ships could crash onto land. Geodesy also helps shipping companies save time and money by shortening their ships' and airplanes' routes and reducing fuel consumption

Geologists, oceanographers, meteorologists, and even paleontologists use geodesy to understand physical processes on, above, and within the Earth. Because geodesy makes extremely accurate measurements (to the centimeter level), scientists can use its results to determine exactly how much the Earth's surface has changed over very short and very long periods of time (Careers in Geodesy, 1 e86)

The Earth's surface changes for many reasons. For instance, its surface rises and falls about 30 centimeters (about 1 foot) every day due to the gravitational influences of the moon and the sun. The Earth's outermost layer, the crust, is made up of a dozen or more "plates" that ride atop a sea of molten rock, called magma, which flows beneath the surface of the Earth.

Plate tectonics is the scientific discipline that looks at how these plates shift and interact, especially in relation to earthquakes and volcanoes. Although these phenomena are violent and usually affect large areas of land, even smaller events, such as erosion and storms, have an impact on shaping the Earth's surface. Geodesy helps us determine exactly where and how much the Earth's surface is changing. ¿F o New York 10" l0' 00' .73' 5l'00'

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The eañh's crusf is made of up separate plates that ride atop a sea of magma. These plates are constantly shifting and interacting. The phenomena that affect the size and shape of the Earth's continents and ocean öasrns are subtle and occur over hundreds of thousands of years. (mage source: G/S-based plate boundaries, ETOPO2 global elevation data).

The History of Geodesy

Throughout history, the shape of the Earth has been debated by scientists and philosophers. By 500 B.C. most scholars thought the Earth was completely spherical. The Greek philosopher Aristotle (384-322 B.C.) is credited as the first person to try and calculate the size of the Eafth by determining its circumference (the length around the equator) He estimated this distance to be 400,000 stades (a stadia is a Greek measurement equaling about 600 feet). With one mile equal to 5,280 feet, Aristotle calculated the distance around the Earth to be about 45,500 miles (Smith, 1988).

Around 250 8.C., another Greek philosopher, Eratosthenes, measured the circumference of the Earth using the following equation:

(360" + q)x (s)

ln this calculation, (s) is the distance between two points that lie north and south of each other on the surface of the Earth. lf you were to draw a line from each of these points to the center of the Earth, the angle formed between them would be q. Obviously, Eratosthenes could not go to the center of the Earth, so he got the angle measurement using the rays of the sun. At noon on the longest day of the year, the summer solstice, the sun shone directly into a deep well at Syene (which is now Aswan, Egypt), casting no shadow.

At the same time in Alexandria, Egypt, he found that the sun cast a shadow equivalent to about 1/50th of a circle or 7.12". Eratosthenes combined this measurement with the distance between Syene and Alexandria, about 4,400 stades. lf we plug these numbers into the above equation, we get: (360"* 7.12") which equals 50; and 50 x 4,400 equals 220,000 stades, or about 25,000 miles. The accepted Compiled by Jib Ahmad, RPLS, CFM - September 2014 Page 3 of 25 Datums in Texas measurement of the Earth's circumference today is about 24,855 miles (Smith, 1988). Given the simple tools and technology that Eratosthenes had at his disposal over 2,000 years ago, his calculations were quite remarkable.

As technology developed, .scientists and surveyors began to use different techniques to measure distance. ln the 16th and 17'n centuries, triangulation started to be used widely. Triangulation is a method of determining the position of a fixed point by measuring the angles to it from two other fixed points that are a known distance apart. Triangulation formed the basis for many national surveys. By the end of the 19th century, majortriangulation networks covered the United States, lndia, Great Britain, and large parts of Europe.

At the end of the 16th century, the Royal Society in London and the L'Academie Royale des Sciences in Paris were founded. Soon they became locked in a battle to determine the shape of the Earth. The French argued that the Earth was prolate, or shaped like an egg. The English, using Sir lsaac Newton's universal theory of gravity and the knowledge that the Earth spun around its axis, thought that the Earth was oblate, or flattened at the poles. To prove their idea, the Academy in Paris staged two expeditions, one to Peru (now Ecuador) at the equator, and the other to the border of Sweden and Finland in the northern hemisphere. Their objective was to measure the north-south curvature of the Earth at each location's latitude and determine whose concept of the Earth's shape was correct. The Academy's efforts proved that Newton was right. The Earth is flattened into the shape of an oblate sphere (Smith, 1988).

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Since the days of ancient mythology, scientists and philosophers have debated the shape of Earth. Since about 500 8.C., the idea that the Eafth was a peffect sphere has dominated most scientific thinking, even though the concept of a flat Eafth may have persisted in some regions for another millennium. Around the end of the 16th century, the idea that the Eañh was a pertect sphere evolved into a radical new idea: that the Eafth was an imperfect sphere. This new way of thinking was initially divided into two major schools of thought. One believed the Earth was egg-shaped (prolate). The other believed the Eafth was flattened at the poles (oblate). The modern concept of a basically oblate Earth was demonstrated to be correct and has spawned many theoretical variations during the last hundred years as geodesy has advanced.

During the last 100 years, geodesy and its applications have advanced tremendously. The 20th century brought space-based technology, making geodetic measurements extremely precise. Today, NAVSTAR Global Positioning System satellites allow scientists to measure changes in the Earth's surface to the centimeter.

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This illustration shows how Eratosthenes actually calculated the circumference of the Eafth. At noon on the summer so/sfice, Eratosthenes measured the length of the shadow cast by a column of known height at Alexandria. With these two lengths, he could solve for the angle between them (q). lf the length of the shadow, and height of the column (h) were proportional to the distance between Alexandria and Syene (s=4,400 sfades,), and the radius of the Eañh, then by calculating the angle on the column (q), he was calculating the same angle formed at the center of the Earth (q). The equation he used to determine the circumference of the Eafth [(360" + q) x (s)] reflects this theory The Elements of Geodesy: The Figure of the Earth

The Earth's shape is nearly spherical, with a radius of about 3,963 miles (6,378 km), and its surface is very irregular. Mountains and valleys make actually measuring this surface impossible because an infinite amount of data would be needed. For example, if you wanted to find the actual surface area of the Grand Canyon, you would have to cover every inch of land. lt would take you many lifetimes to measure every crevice, valley, and rise. You could never complete the project because it would take too long.

To measure the Earth and avoid the problems that places like the Grand Canyon present, geodesists use a theoretical mathematical surface called the ellipsoid. Because the ellipsoid exists only in theory and not in real life, it can be completely smooth and does not take any irregularities - such as mountains or valleys -- into account. The ellipsoid is created by rotating an ellipse around its shorter axis. This matches the real Earth's shape, because the earth is slightly flattened at the poles and bulges at the equator.

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While the ellipsoid gives a common reference to geodesists, it is still only a mathematical concept. Geodesists often need to account for the reality of the Earth's surface. To meet this need, the geoid, a shape that refers to global mean sea level, was created. lf the geoid really existed, the surface of the Earth would be equalto a level in between the hightide and low{ide marks.

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Because the sufface of the Eafth is irregular and complex. geodesrsfs use simplified mathematical models of the Earth for many applications. The simplest model of the Eañh is a sphere. A much more complex model of the Earth is the geoid, used to approximate mean sea level. Even the geoid is a simplified model, however, when compared to actual topographic relief, as shown in this image of four figures of the Eafth (Grand Canyon).

Although a geoid may seem to be a smooth, regular shape, it isn't. The Earth's mass is unevenly distributed, meaning that certain areas of the planet experience more gravitational "pull" than others. Because of these variations in gravitational force, the "height" of different parts of the geoid is always changing, moving up and down in response to gravity. The geoidal surface is an irregular shape with a wavy appearance; there are rises in some areas and dips in others (Geodesy for the Layman, 1984).

The Elements of Geodesy: Datums

Datums (sets of data) are the basis for all geodetic survey work. They act as reference points in the same way that starting points do when you give someone directions. For instance, when you want to tell someone how to get to your house, you give them a starting point that they know, like a road or a building. Geodesists and surveyors use datums as starting or reference points when they create maps, mark off property boundaries, and plan, design and build roads, bridges, and other structures.

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Another way to think about a datum is as a set of information that acts as a foundation for other data. For example, when a skyscraper is about to be built, the construction team must first pour the foundation. Without this element, the skyscraper would be unstable and unsafe. This is the same concept as a datum. While a datum is a mathematical and geometric concept, it acts like the concrete foundation of a skyscraper. Once the foundation is set, the construction workers can build on top of it, creating the building's structure. After the building is complete, otfices or apartments can be created inside the building. lf the structure is an apartment building, its tenants can bring in furniture and decorate as they please. Although the foundation of the building probably isn't the first thing on the minds of the tenants, without it, the building would not be a safe place to live.

ln geodesy, two main datums create the foundation for navigation and transportation in the United States. These datums - called the horizontal and vertical datums -- make up the National Spatial Reference System (NSRS). Geodesists, surveyors, and people interested in precise positioning use the NSRS as their foundation for reference.

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One way to think about a datum r,s as a set of spatial information that acts as a foundation for other data, just like concrete acfs as a foundation for the structure of a building, including all the furnishings and decorations inside. Without a datum there would be nothing to securely suppott other spatial information, such as digital elevations, land use, or population. There are two types of datums, horizontal and vertical, that make up the National Spatial Reference Sysfem úVSRS).

Compiled by Jib Ahmad, RPLS, CFM - September 2014 PageT of 25 Datums in Texas The Elements of Geodesy: The Horizontal Datum

At its most basic level of definition, the horizontal datum is a collection of specific points on the Earth that have been identified according to their precise northerly or southerly location (latitude) and easterly or westerly location (longitude) (National Geodetic Survey, 1986). To create the horizontal datum, or network of horizontal positions, surveyors marked each of the positions they had identified, typically with a brass, bronze, or aluminum disk or monument. These markers were placed so that surveyors could see one marked position from another. To maximize the line-of-sight between monuments, they were usually set on mountaintops or at high elevations. When monuments were set on flat land, towers were built above them to aid surveyors in locating them.

To "connect" the horizontal monuments into a unified network, or datum, surveyors have used a variety of methods, including triangulation. As technology has improved, surveyors now rely almost exclusively on the Global Positioning System (GPS) to identify locations on the Earth and incorporate them into existing datums. ln 1927 the U.S Coast and Geodetic Survey, the predecessor of the National Geodetic Survey, "connected" all of the existing horizontal monuments together and created the North American Datum of 1927 (NAD 27). This datum was used extensively during the next60 years as the primary reference for horizontal positioning. ln 1983, NAD 27 was adjusted to remove inaccuracies and to correct distortions. The new datum, called NAD 83, is the most commonly used horizontal positioning datum today in the United States.

One application of the horizontal datum is monitoring the movement of the Earth's crust. This type of monitoring is often used in places like the San Andreas Fault in California where many earthquakes occur. Extending from northern California south to San Bernardino, the San Andreas Fault is where two plates of the Earth's crust meet. The fault is approximately 800 miles long and extends 10 miles below the Earth's surface. By looking at the movement of monuments in the horizontal datum, geodesists can determine just how much the surface of the Earth has moved after an earthquake (Schultz and Wallace, 1ee7).

Brass monuments permanently affixed in concrete or surrounding bedrock indicate accurate geodetic reference positions within the National Geodetic Survey's horizontal and/or veftical datums.

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The Elements of Geodesy: The Vertical Datum

The vertical datum is a collection of specific points on the Earth with known heights either above or below mean sea level. Near coastal areas, mean sea level is determined with a tide gauge. ln areas far away from the shore, mean sea level is determined by the shape of the geoid.

Similar to the survey markers used to identify known positions in the horizontal datum, round brass plates mark positions in the vertical datum. The traditional method for setting these vertical benchmarks is called differential leveling. This method uses a known elevation at one location to determine the elevation at another location. As with horizontal datums, the advanced technology of GPS has almost completely replaced this classical technique of vertical measurement. ln 1929, the NationalGeodetic Survey (NGS) compiled allof the existing vertical benchmarks and created the National Geodetic Vertical Datum of 1929 (NGVD 29). Since then, movements of the Earth's crust have changed the elevations of many benchmarks. ln'1988, NGVD 29 was adjusted to remove inaccuracies and to correct distortions. The new datum, called the North American Vertical Datum of 1988 (NAVD 88), is the most commonly used vertical datum in the United States today.

One of the main uses of the vertical datum is to measure rates of subsidence, or land sinking. ln Louisiana, for example, large areas of land are rapidly sinking This is the result of development, coastal erosion, and high population levels. ln many areas, the only way to escape an incoming hurricane is to follow specific hurricane evacuation routes. lf state and local officials do not have accurate elevation information about these routes, residents trying to leave during an emergency might get trapped in fast- rising water.

By referencing the vertical datum, officials can determine the true elevation and position of the hurricane evacuation routes, as well as how much they have sunk over past decades. For example, in Plaquemines Parish, Louisiana, the main hurricane evacuation route, Highway 23, and the surrounding levees are subsiding by one-quarter to one-half inch per year à.s .J åls¿IÊ Ev.at.t¡on R oule Su rurye rl J.Jrle ¿009 ãeJlc C.lirsrc Rou1e23 L ìt -t !!rlc J 9,ú I \, - tÀs i:;

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Highway 23 is the main hurricane evacuation route for the entire state of Louisiana. Veftical benchmarks have been critical in tracking the rate of subsidence in the area and allowing officials to develop up to date emergency evacuation plans.

The Elements of Geodesy: Gravity

Gravity is the force that pulls all objects in the universe toward each other. On Earth, gravity pulls all objects "downward" toward the center of the planet. According to Sir lsaac Newton's Universal Law of Gravitation, the gravitational attraction between two bodies is stronger when the masses of the objects are greater and closer together. This rule applies to the Earth's gravitational field as well. Because the Earth rotates and its mass and density vary at different locations on the planet, gravity also varies.

One reason that geodesists measure variations in the Earth's gravity is because gravity plays a major role in determining mean sea level. Geodesists calculate the elevation of locations on the Earth's surface based on the mean sea level. So knowing how gravity changes sea level helps geodesists make more accurate measurements. ln general, in areas of the planet where gravitational forces are stronger, the mean sea level will be higher. ln areas where the Earth's gravitatíonal forces are weaker, the mean sea levelwill be lower.

To measure the Earth's gravity field, geodesists use instruments in space and on land. ln space, satellites gather data on gravitational changes as they pass over points on the Earth's surface. On land, devices called gravimeters measure the Earth's gravitational pull on a suspended mass. With this data, geodesists can create detailed maps of gravitational fields and adjust elevations on existing maps. Gravity principally affects the vertical datum because it changes the elevation of the land surface (Geodesy for the Layman, 1984).

Compiled by Jib Ahmad, RPLS, CFM - September 2014 Page l0 of 25 Datums in Texas lmagine if all of the Eafth topography, mountains and valleys were scoured off leaving a continuous world's ocean completely at rest, without the effects of currents, weather and tides. The effect of the Earth's gravity on this hypothetical world mean sea level is represented by the geoid. However, because the Eafth's gravity is not equal in all places, this hypothetical ocean is not pertectly smooth. The strength of the Eafths' gravity and consequent effect on the shape of the geoid is represented by color variations in this image.

The National Spatial Reference System

All of the elements of geodesy are joined together in the National Spatial Reference System (NSRS). For almost 200 years, the National Geodetic Survey (NGS) and its predecessors have been using geodesy to map the U.S. shoreline, determine land boundaries, and improve transportation and navigation safety. NGS evolved from the Survey of the Coast, an agency established by Thomas Jefferson in 1807. The creation of the United States' first civilian scientific agency was prompted by the increasing importance of waterborne commerce to the fledgling country. As the nation grew westward, NGS's mission began to include surveys of the North American interior.

With numerous surveys being conducted simultaneously across the growing nation, the surveyors needed to establish a common set of reference points. This would insure that surveyors' maps and charts, which often covered hundreds of miles, would align with each other and not overlap. The common set of reference points they used were the benchmarks from the horizontal and vertical datums. Today, the complete set of vertical and horizontal benchmarks for the United States is known as the NSRS. This defined group of reference points acts as the foundation for innumerable activities requíring accurate geodetic information.

Think of it this way: When construction workers begin to build, they have to be sure that the area where they are building is free from dangerous power lines. The construction team will have to find out where the power lines are and make sure they are not building on top of them. To ensure success, the team needs to know the coordinates of the building site and of the local power lines. The NSRS provides a framework for identifying these coordinates. The team can then compare the two sets of coordinates and make sure they do not overlap.

To identify the benchmarks in the NSRS, NGS has traditionally placed markers, or permanent monuments, where the coordinates have been determined. These markers are brass or bronze disks (metals that sustain weathering) and are set in concrete or bedrock. Each marker is about 9 centimeters wide and has information about NGS printed on its surface.

With the advent of the Gfobaf Positioning System (GPS), NGS began to use different kinds of markers. These are made from long steel rods, driven to refusal (pushed into the ground until they won't go any farther.) The top of each rod is then covered with a metal plate. This method ensures that the mark won't move and that people can't destroy or remove it. After tying these marks into a specific horizontal or vertical datum, the mark can be included in the NSRS database. Once the coordinates of the mark are entered into this database, they are available for anyone to use.

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A United Sfafes triangulation network map from 1937 Developing a network of areas based on manual surveys using triangulation was a precursor to today's modern National Spatial Reference System, which rs oyerseen and continuously monitored by the Global Positioning Sysfem.

Do you know where you are? - The Global Positioning System

Using the Global Positioning System (GPS), every point on Earth can be given its own unique address -- its latitude, longitude, and height. The U.S. Department of Defense developed GPS satellites as a strategic system in 1 978. But now, anyone can gather data from them For instance, many new cars have a GPS receiver built into them. These receivers help drivers know exactly where they are, and can help them from getting lost.

GPS is a constellation of satellites that orbit approximately I 1,000 miles above the Earth and transmit radio wave signals to receivers across the planet. By determining the time that it takes for a GPS satellite signal to reach your receiver, you can calculate your distance to the satellite and figure out your exact location on the Earth. Sound easy? ln fact it is a very complicated process. For the GPS system to work, you need to have incredibly precise clocks on the satellites and receivers, and you must be able to access and interpret the signals from several orbiting satellites simultaneously. Fortunately, the receivers take care of all the calculations.

Let's tackle the distance calculation first. GPS satellites have very precise clocks that tell time to within 40 nanoseconds or 40 billionths (0.000000040) of a second. There are also clocks in the GPS receivers Radio wave signals from the satellites travel at 186,000 miles per second To find the distance from a satellite to a receiver, use the following equation: (186,000 mi/sec) x (signal travel time in seconds) = Distance of the satellite to the receiver in miles.

Knowing the distance of your GPS receiver to a single satellite is useful, but it will not provide you with enough information to determine your exact position on the Earth. For that, you need to simultaneously access the signals from four satellites. By calculating its distance from three satellites simultaneously, a GPS receiver can determine its general position with respect to latitude, longitude, and elevation.

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You may wonder why the GPS receiver needs a fourth satellite. Essentially, it provides for even greater precision. To accurately calculate the distance from the GPS receiver to each of the three orbiting satellites, you must know the precise radio signal transmission and reception times. To do this, the clocks in the satellites and the clocks in the receivers must be perfectly synchronized. A mismatch of as little as one millionth of a second between the clock in the satellite and the clock in the receiver could translate into a positioning error of as much as 900 feet (Herring). The atomic clocks in the satellites are extremely accurate, but the clocks in the GPS receivers are not, which creates a timing error. The signal from the fourth satellite is used to adjust for the receiver clock error and calculate the receiver's correct position. So, the receiver needs at least four satellite signals to calculate a position. But if the receiver can access more satellite signals, it will calculate a more accurate position (Tinambunan).

It takes four GPS safe//ifes to calculate a precise location on the Eafth using the Global Positioning Sysfem: three to determine a position on the Eafth, and one to adjust for the error in the receiver's clock. lf you were 15,000 miles from only one satellite (satellite with red sphere), you could be anywhere on an imaginary red sphere that has a radius equal to the number of miles from the satellite (15,000 miles).

Taking it to the next level: CORS and GIS ln a field of study that is thousands of years old, GPS represents a quantum leap in geodesy. As advanced as GPS technology is, most commercially available GPS receivers are only accurate within several meters. Considering that the Earth is almost 25,000 miles in circumference, the difference of a few meters may not seem important. This level of accuracy may be adequate for a hiker in the woods or someone driving a car. But there are many scientific, military, and engineering activities that require much higher levels of positioning accuracy - often to within a few centimeters or less!

To provide measurements at this level of accuracy, NGS developed the Continuously Operating Reference Stations (CORS) network. CORS is a network of hundreds of stationary, permanently operating GPS receivers throughout the United States. Working 24 hours a day, seven days a week, CORS stations continuously receive GPS radio signals and integrate their positional data into the National Spatial Reference System. This data is then distributed over the lnternet. After logging onto the CORS Web site, users can determine the accuracy of their coordinates to the centimeter. This system

Compiled by Jib Ahmad, RPLS, CFM - September 2014 Page l3 of 25 Datums in Texas has been especially useful in assessing the integrity of buildings and bridges in areas that are geologically active or have been impacted by natural disasters such as hurricanes or floods.

Another powerful tool that has evolved along with GPS technology is the Geographic lnformation System (GlS). A GIS is comprised of three parts: spatial information, special software, and a computer. These components work together to provide a digital platform for viewing and processing layers of spatial information.

A GIS assembles information from a several of sources, including ground surveys, existing maps, aerial photos, and satellite imagery. ln a GlS, specific information about a place, such as the locations of utility lines, roads, streams, buildings, and even trees and animal populations, is layered over a set of geodetic data. Using special software, regional planners and scientists can examine the layers individually or in various combinations to improve traffic flow, merge construction with utility systems, develop around environmentally sensitive areas, and protect the public from potential natural disasters. Because a GIS stores data digitally, information can be quickly and economically updated, easily reproduced, and made widely available. ln fact, because of its power and speed, GIS technology is doing most of the cartographic (mapmaking) work that, in the past, was laboriously done by hand on paper charts and maps.

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The Continuously Operating Reference Sfafions ICORS) system is a network of sfafions throughout the United Sfafes and its territories that continually record GPS slgnals, and then provide the data fo GPS users over the lnternet.

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Information about a place in a G/S rs sfored in several separate but overlapping layers that represent utility lines, buildings, roads, aerial photography, and topography.

References: http://www.oceanservice.noaa.gov/education/kits/geodesy/geo11_ref.htm19l21l2007 1233.17 PM

Careers in Geodesy. 1986. The American Geophysical Union Section of Geodesy. Washington, DC American Geophysical Union. 30 pp.

Geodesy for the Layman. 1984. Washington, DC: Defense Mapping Agency. Online at: http://www. nqs. noaa.sov/PUBS Ll B/Geodesv4LaymanlTR8OO03A. HTM.

Geodetic Glossary. 1986. Rockville, MD. National Geodetic Survey. 266 pp.

Herring, Thomas. 1996, February. The Global Positioning System. Scientific American, 44-50.

Hurn, Jeff. 1989. GPS: AGuidetothe NextUtility. Sunnyvale, CA: Trimble Navigation Ltd.76 pp.

Schultz, Sandra S. and Robert E. Wallace. 1997. The San Andreas Fault. United States Geological Survey. Online at:

Smith, J.R. 1988. Basic Geodesy. Rancho Cordova, CA: Landmark Enterprises. 135 pp. Compiled by Jib Ahmad, RPLS, CFM - September 2014 Page 15 of 25 Datums in Texas

Tinambunan, D. 2003. How do GPS devices work? ScientificAmerican.com. Online at: http.//www.sciam.com/askexpert_question.cfm?ArticlelD=000349D4-D6FC-1CFC- 93F6809 EC5880000&catl D=3.

Vanicek, Petr. 2000. An Online Tutorial in Geodesy. Academic Press. Online at: http://ei nstei n. g ge. u nb.caltutorial/tutorial. htm.

Beyond GEOID12: Implementing a ltlew Vertical Datum for North America

Daniel R. ROMAN, Neil D. WESTON, UNITED STATES

Key words: Positioning, Heights, GPS/Leveling, Geoid, Vertical Datum

SUMMARY

The National Geodetic Survey (NGS) is responsible for maintaining both the horizontal and vertical datums within the U.S. National Spatial Reference System OISRS), which are the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88), respectively. NGS periodically produces hybrid geoid height models that transform between these datums to facilitate GPS/leveling in surveying as well as other engineering and scientific activities. The GEOIDI2 model represents the latest effort in this series. However, both NAD 83 and NAVD 88 have signihcant systematic problems, which the current hybrid geoid height models faithfully replicate. While the datums remain internally consistent (i.e., precise) they are inconsistent with other reference systems at the meter level (i.e., not accurate). Comparisons at tide gauges and with global satellite gravity field models demonstrated a meter level cross-continent trend in NAVD 88 likely due to accumulated leveling effors in the adjustment that created it. Likewise NAD 83 is known to have a 2.2 meter offset from IGS 2008. The Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project was started to realize a new vertical datum that is both accurate and precise. The aim of the project is to produce a gravimetric geoid height model that is accurate to cm- level and can be combined with an improved geometric reference frame to produce similarly improved physical heights, removing entirely the need to have "hybrid" geoid models which absorb systematic datum effors. Several factors are critical to ensuring this works. In anticipation of its adoption in 2022 at the completion of GRAV-D, an optimum geopotential surface (geoid) was recently selected based on comparisons with tidal bench marks around Canada and the United States and some portions of the Caribbean. A geopotential value of 62,636,856.00 m'ls'best fit available data. This value is the same as that adopted by the International Astronomical Union (lAU) & the International Earth Rotation and Reference Systems Service (IERS) and is one of many that have been offered as the best representative of global mean sea level (MSL). Comparisons all around the North American continent with tide gauges, satellite altimeter measurements of the ocean surface, and models of ocean height variability all support adoption of this number as the best estimate of MSL for North America and future vertical reference systems defined for the United States and Canada. Canada will be adopting this value and a geoid height model based on it as their official verticaf datum in2013 while the United States will follow suit in 2022. The United States continues Compiled by Jib Ahmad, RPLS, CFM - September 2014 Page 16 of 25 Datums in Texas to collect aerogravity to remove systematic effors in the terrestrial gravity data holdings to ensure that a cm-level of accuracy is achieved. This is on track and should be accomplished as planned in 2022 with a ne\¡/ vertical datum realized by a gravimetric geoid height model and GNSS observations.

Beyond GEOlDl2: lmplementing a New Vertical Datum for North America

Danie| R. ROMAN, NeiI D. WESTON, UNITED STATES.

I.INTRODUCTION

The National Geodetic Survey (NGS) is a program office within the National Ocean Service of the National Oceanic and Atmospheric Administration. NGS is responsible for maintaining both the geometric and geopotential datums within the U.S. National Spatial Reference System (NSRS). The focus of this paper is on the ongoing development of geoíd height models as a mechanism for accessing the geopotential datum through use of GNSS technology. Geoíd heights are essentially heights above the ellipsoid to the geoid (the geopotential surface that approximates MSL). Many such models exist both at global and regional scales, and they are constantly being refined.

The two most recent regional models for the United States are USGG2012 (the U.S. Gravimetric Geoid of 2012) and GEOlDl2 (which is the latest in a long series of "hybrid" geoid models). USGG2012 is in lGS2008 (epoch 2005.0) to be consistent with the coordinate system used for the satellite orbits, CORS sites, and OPUS solutions. USGG2O12 is the best estimate of global MSL based on comparisons with satellite altimetry & tide gauges but with a focus on North America. Derived geoid heights are applied to lGS2008 coordinates to obtain geopotential heights that most accurately reflect heights and changes in height. GEOlDl2 is in NAD 83 (201 1) epoch 2010.0, which is the official geometric datum of the NSRS in which the passive control (bench marks) are published as a result of the National Adjustment of 2011 (NA2011) Project. GEOlDl2 heights provide the separation between NAD 83 and NAVD 88, the official vertical datum of the NSRS.While NAD 83 and NAVD 88 remain the official components of the NSRS for now, they are slated for replacement in 2022. NAD 83 has a known 2.2 m geocenter offset from most recent ITRF models (Snay 2003) and NAVD 88 has a 1-2 meter offset from satellite-determined EGM's (Wang et al. 2012). Hence, a geometric datum will be adopted in 2022 that will likely be derived from the then most current realization of the lnternational Terrestrial Reference System. Also, a geopotential datum will be adopted that will contain a geoid height model tied into that same geometric datum. This paper will focus on the steps that are being implemented to adopt such a geoid model.

NGS is an active member of the IAG's Sub-Commission 2.3.c: the North American Geoid. The intent of IAG SC 2.3.c is to develop a regional geoid height model suitable to all countries in the region or at least to establish a common set of data for development of models that are consistent to an acceptable tolerance level (cm-level). NGS has collaborated with cadastral agencies in Canada, Mexico (the current chair), most countries throughout Central America, several countries in the Caribbean region, and the Geocentric Reference System for the Americas (SIRGAS) committee in planning such a model. NGS' efforts, in particular, are focused on the development of a harmonic model that describes all aspects of the gravity field to include geopotential numbers. With geopotential numbers, any type of height may generated and be consistent with the local gravity field values.

To ensure such consistency, several layers of collaboration and data collection are required. NGS is engaged with ESA and other scientific agencies involved in the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and Gravity Recovery And Climate Experiment (GRACE) satellite gravity models as well as in GGOS to ensure that the North American Vertical Reference System (a working name for the 2022 geopotential datum) will be consistent with the eventual World Height System. Additionally, comparisons are made at tide gauges throughout North America to ensure consistent ties to MSL values observed regionally and not just in a global least squares sense. A comprehensive aerogravity campaign was instituted by NGS to ensure consistency and accuracy and to link the global satellite gravity models to the millions of terrestrial data points. Finally, all of these overlapping data sets must be melded to provide a consistent, accurate, seamless, gravity field model suitable for cm-level Compiled by Jib Ahmad, RPLS, CFM - September 2014 Page 17 of 25 Datums in Texas height determination in conjunction with GNSS. This paper will first address the development of USGG2O12 and selection of the geoid surface based on tide gauge comparisons. lt then covers the development of GEO|D12, and finally addresses the outlook for future work and impacts on other programs.

2. UNTTED STATES GRAVTMETRTC GEOTD 2012 (USGG2012)

NGS has produced a series of gravimetric geoid height models for about 20 years (Wang el al. 2012, Roman eta|.2004, Smith and Milbert 1999). The process fordeveloping these has become more refined using increasingly accurate long wavelength models from satellite gravity missions such as GRACE (Tapley et al. 2004) and more GOCE (Drinkwater et al. 2006). These models were blended into NGA's EGM2008 model (Pavlis et al. 2008) to produce a consistent reference model for calculating gravimetric geoids in the Conterminous United States (CONUS), Alaska, Hawaii, Puerto Rico & the U.S. Virgin lslands (PRVI), American Samoa, Guam, and the Commonwealth of the Northern Marianas lslands (cNMr)

Terrestrial and shipborne gravity are available for all of these regions and DTUl0 altimetric anomalies (Cheng and Andersen 2011) were used to supplement data in littoral regions surrounding the landmasses. No aerogravity was used pending ongoing testing and evaluation. Future models will likely include such data. These included gravity data and the reference model were combined using a modified kernel that removed long wavelength disagreements between the terrestrial data and the satellite data in order to force a fit to the long wavelengths provided from GRACE and GOCE. The shorter wavelengths of the terrain were also taken into account. EGM2008 accounts for the effects of the terrain through about five arcminute (5') or about 10 km scale. SRTM 3" (90 m) data are available globally and a Residual Terrain Model (RTM) between 3" and 5' accounted for much of the high frequency variability seen in the residual gravity signal. This removed short wavelength variability that appeared to be noise, smoothed the residual signal, and resulted in much greater accuracy in the final model.

The techniques employed to perform these steps largely follow the blueprint laid out in Wang etal. (2012) for the development of USGG2O12. While no significant advances were made in technique, one aspect of USGG2012 is worthy of further mention. The geopotential surface selected as the geoid (datum surface or W0) for this model was chosen as 62,636,856.00 m2ls2. This value defines which equipotential surface will serve as the geoid and thus the datum from which heights are measured. Geopotential numbers are defined as the change in geopotential value from the chosen W0. The geoid serves as the zero surface from which heights are determined to be above and even below. Choosing this value now has taken on added significance in view of the ongoing collaboration between the United States and Canada to determine a common surface for defining heights. Canada needed to determine a value this year for their gravimetric geoid, which will serve as their official vertical datum next year. The United States will follow suit in 2022, after significant data problems are addressed via the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project (GRAVD Science Team 2011). Until that time, hybrid models such as GEOlDl2 (Section 4) will continue to serve as the primary mechanism for determining orthometric heights from GNSS.

3. DETERMINING THE DATUM SURFACE FOR A NORTH AMERICAN GEOID

USGG2012 is required to make GEOlDl2 and provided amplifying information between control points used in GEOlDl2. USGG2012 also provided a check on progress towards a common geoid model with the Canadians to ensure the greater likelihood of a unified North American Vertical Datum as was envisioned for NAVD 88. Tide stations around Alaska, Canada, CONUS, and PRVI were all evaluated to estimate the most optimal geopotential value for selection of the geoid surface for a future vertical datum (Figure 1).

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85' 80' 620 'ä:ìs 61 5 75' ¿qÞ 61.0 60,5 70' 60.0 -øF2 59.5 65 59.0 60 58.5 I 58,0 55' rr! 57.5 .r\ 57,O 50' I¡ 56.5 56,0 45 s5.5 55.0 40" 545 35' 540 53.5 30' 530 525 25', 52,O 51.s 20' Þ 51,0 't5' 50,5 500 180' 1 200" 210' 220' 240" 250' 260" 280' 290' 300" 310' 30 0/. fipts=2f I o 20 oE "/. ltêan=56.85 gf STD=2-81 L 10 "/"

o o/. 50 55 60 W0-62,636,800 at TGs (m2ls2)

Figure 1 Geopotential values determined for tide gauges around Alaska, Canada, CO/VUS, and PRVI. The effects of SSf can be seen especially along the Atlantic Coasf. A constant value of 62,636,800 was removed from all numbers. Based on this comparison, the average W0 should be 62,636,856.85 mUs2.

GNSS observations at these tide stations placed the local Mean Sea Level (MSL) value into a geometric reference system. However, quite a bit of variability remained in the original observations due to the effects of Sea Surface Topography (SST) or Mean Ocean Dynamic Topography. Ocean currents, such as the Gulf Stream of the Labrador Current, not only provide a constant deviation of mean sea level from an equipotential surface, they also move warmer (less dense) and colder (more dense) waters along the shoreline. The combined effect of these produces upwards of a meter trend along the Atlantic shoreline. Additionally, an approximate 50 cm difference exists in the ocean heights (relative to the geoid) between the Atlantic and Pacific Oceans. SST models by oceanographers (Foreman et al. 2008, Thompson and Demirov 2006) are shown in Figure 2 and greatly reduce the difficulty of determining the value of the optimal geopotential surface from MSL observations.

'a

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,6-15 (¿ r! \2 )1 )t it !ì rJ )4 rs o6 ú/ de : 'ìl r.f... M¡1. 'x '6) SSr -.erS-Mio,n Ía)¡FX l,ltrñr

Figure 2 SS.l- Models for the Pacific (Foreman et al. 2004) and Atlantic (Thompson-Demiorov 2006). SST models account for ocean surface variations away from a global norm (local vs. global MSL). Compiled by Jib Ahmad, RPLS, CFM - September 2014 Page lg of 25 Datums in Texas

Figure 3 shows the revised geopotential values at tide gauges with the SST taken into account. The value of 62,636,856.00 m2ls2 best fits the North American region and provides the best estimate of MSL to serve as a datum. Coincidentally, this value was originally published by Burða et al. (2007) from an analysis of global data and was adopted by the IAU and IERS. lt, therefore, provides a value vetted for global models but matched to regionaltide gauges.

85 80' 540 75 4}] s75

70 570 565 65 "2';:2 560 60 I s55 55 -l\ 550 50 545 45 540 40' 535 35 53.0 30 525 25 520 20 Þ 51 5 15 51 0 180 1 200' 210' 220' 250 260' 280' 290 300 310 30% þts-188 c 20y" Mean=56.00 o q- STD=I.429 o L 10 "/"

o o/" 50 55 60 W0-62.636.800 al TGs - SST lm2lsq

Figure 3 Comparisons at tide gauges around Canada & the United Sfafes. Colors scale the geoptential values (W0) of local MSL at the tide gauges taking into accounf Sea Surface Topography. A whole value of 62,636,800 was removed from the scale. The best value then for appoximating MSL is 62,636,856.00 m2/s2.

Since the SST models didn't span the Arctic and other outlying (and largely unoccupied) regions, the scope of the stations was somewhat reduced from Figure 1 to Figure 3. The likely effect of adding those stations back in would be to increase the mean value. However, the value needed is intended for all of North America. While GNSS observations on tide gauges in Mexico and regions of Central America and the Caribbean are not readily available, the expected values there would lower the mean value. Hence, regions that have been excluded should offset and produce a value consistent with that selected. For those reasons then, the value of 62,636,856.00 m2ls2 was officially adopted by Canada and the United States for gravimetric geoid models in 2012 and the immediate future (with its adoption in 2022 remaining open to re-evaluation).

4. GEOtDl2

The GEOlDl2 model represents the latest in a series of models designed to transform between NAD 83 and NAVD 88. Updates are necessary as the intent of these models is to ensure that they best match the coordinates on those datums for bench mark control stations maintained by NGS. Any changes or updates to the values in the database require that new models be produced. Such models represent a blend of the underlying gravimetric geoid through the control points defined at the bench marks, where both ellipsoidal and orthometric heights are required information. The resulting model is a blend or hybrid that reflects the systematic differences between NAD 83 and NAVD 88 realized through the bench marks and using the information in the gravimetric geoid model to provide detail between the control points.

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Hence, the nature and distribution of the control points is critical. The GPS-derived ellipsoidal heights on leveled Bench Marks available in2012 (GPSBM2012) represent the intersection of nearly 500,000 bench marks in NAVD 88 with the nearly 70,000 bench marks with NAD 83 coordinates. The intersection is only around 23,000 points, which is not surprising when one considers that most NAVD 88 bench marks were installed in locations very unwelcoming to tripods and GNSS surveying in general (against walls, poles, posts and in bridge culverts). With the advent of GPS and further GNSS technologies, a more concerted effort has been made to occupy more leveled bench marks, as possible.

Additionally, the distribution of the points remains non-uniform with about half of the points in GPSBM2012 being located in only five of the 48 states in CONUS. Western states, in particular, are very poorly represented and often have significant subsidence problems at the few available points due to groundwater & oilextraction and mining activities.

As in 2009, the GPSBM2012 were vetted by a larger group of NGS employees with surveying experience and knowledge of potential problem marks not obvious from examination of the database. NGS maintains a Regional/State Adviser program to ensure that there is a link between NGS HQ actions, models, and planning and what is desired or implemented at the various state agencies responsible for geodetic applications (e.9., state Department of Transportation).

5. USE OF ONLTNE USER POSTTTON SERVTCE DATABASE (OPUS-DB)

For this reason, over 700 supplemental control points available from the OPUS-DB (Weston et al. 2007) were examined to provide increased density. This collection of archived GNSS observations is input by surveyors. A smaller subset of these points was made on existing leveled bench marks previously not occupied by GNSS (OPUSDBBMl2). This amplifying information fills in between the existing GPSBM2012 controlto further refine hybrid geoid models such as GEO|Dl2 (Roman and Weston 2011). Figure 4 shows their distribution and highlights the fact that only some states have participated in this effort. Given that this can be a targeted effort, use of OPUS-DB is expected to grow in future models and bridge gaps that are currently interpolated producing systematic errors thought to be in the dm range. These new data provided significant improvements in Texas but were less helpful in other states.

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Figure 4. Locations of 709 points stored in the OPUS-DB and where no geometric coordinates exist in the NGS dafabase. Ihese GNSS observations on leveled bench marks (OPUSDBBMI2) supplement GPSBM2012. Texas is an example of where this supplemental information created dm-level changes.

These control points (GPSBM2012 plus OPUSDBBMl2) were used to develop a conversion surface that reflects the changes needed to transform the USGG2O12 grid into GEO|D12. Multiple matrices were inverted through Least Squares Collocation to develop this surface. Additional information was available both in Canada and Mexico where leveled bench mark heights above NAVD 88 also exist, although they were not formally adopted by those countries. Similar models were developed for all U.S. regions with CONUS and Alaska being on the NAVD 88 datum and outlying territories being on their own respective tidaldatums.

6. OUTLOOK

For the next decade, NGS will continue to produce both gravimetric and hybrid geoid models as they have in the past. Steady improvements are expected in the development of gravimetric geoids with the goal being to achieve cm-level accuracy in an absolute sense. A common geopotential value has already been selected jointly with the Canadians. Available terrestrial data have begun to be analyzed and adjusted to be consistent with satellite models of the Earth's gravity field. Aerogravity, gravity collected from an airborne platform and sensor in flight, serve to bridge the gap between the satellite-based global models and the terrestrial data. ln turn, rigorous theory is being implemented to take this improved gravity data to meld it into a geoid height model suitable for use as a vertical datum. The Gravity for the Redefinition of the American Vertical Datum (GRAV-D) Project is intended as the means of accomplishing all of this. The current status of aerogravity flights is given in Figure 5.

Figure 5 Extent of aerogravity collections as of Spring 2012. Map Key: Green: Available data and metadata; Blue: Data being processed, metadata may be available; Orange: Data collection undervvay; and White: Planned for data collection.

To insure data integrity, aerogravity is flown in a consistent manner following rigorous standards. Base station ties are made with an A-10 absolute gravimeter, concurrentwith the aerogravity survey, and not made to older gravity points that may have changed. ln turn, the aerogravity and satellite models are merged to produce an improved reference model to detect and mitigate any systematic errors in the terrestrial data due to either geophysical changes (e.9., subsidence) or bad base station ties (e.9., a survey bias). Compiled by Jib Ahmad, RPLS, CFM - September 2014 Page 22 of 25 Datums in Texas

Gravimetric geoid models are being developed that include this information and will be posted on the GRAV-D page (http://www.geodesy.noaa.gov/GRAV-D/) as experimental models to highlight the expected vertical datum values in 2022. As these models become more refined, they may be adopted as future USGG2Oxx models, though that is subject to further testing against external metrics such as tide gauges. ln 2022, however, USGG2022 will effectively become GEO|D22.

There will continue to be follow-on hybrid geoid models such as GEOlDl2 through 2022. At that time, no further hybrid models will be posted as official products. However, a conversion surface will continue to be generated (as was required to make GEOlDl2 from USGG2012). This surface will effectively transform between the new datum surface and NAVD 88 for as long as is required; much as VERTCON provides a transformation between NGVD 29 and NAVD 88. Note though that bench marks will no longer be vigorously maintained and that the network of passive marks will eventual become obsolete (or "secondary access" to the vertical datum, meaning that the reliability of passive marks will be entirely dependent on the user community's desire and effort to re-survey points of interest to them). At that point, future gravimetric geoid models and GNSS surveying will serve as the primary means of accessing the NSRS.

7. SUMMARY

NGS is charged with defining, maintaining and providing access to the National Spatial Reference System. The existing datums in the NSRS are NAD 83 (geometric) and NAVD 88 (geopotential). GEOlDl2 is based on a larger pool of control data (GPSBM2012) than previous models and was supplemented by OPUSDBBMl2 observations on leveled bench marks. lt serves as the official model because it transforms between the otficial datums.

The underlying gravimetric geoid model, USGG2012 was built first and used to underpin the GEO|Dl2 model. lt was determined from satellite data and terrestrial observations with no aerogravity included. This model was defined as the best current representation of the geopotential surface (W0) with the value of 62,636,856.00 m2ls2. This surface was selected as the best fit through tide gauges through Canada and the United States when the effects of Sea Surface Topography were taken into account. lt, therefore, provides the best estimate of MSL in all of North America - a suitable datum for determining heights. lt also is the value adopted by many others as an estimate of global MSL. This surface was adopted by Canada and will be used to define their CGG2013 model, which will become their official vertical datum definition next year. The United States will follow suit with a similar adoption of a gravimetric geoid height model as the means for determining heights presumably about2022.

Between now and then, the GRAV-D project will collect aerogravity to bridge the gap between global gravity field models from satellite missions and terrestrial data, and then use that data to develop a geoid height model that is determined to be cm-level accurate from comparisons to other external metrics such as tide gauges. When that model is complete and in place for all U.S. states and territories, it will be implemented with a target date of 2022.

ACKNOWLEDGMENTS

Special thanks Xiaopeng Li of Earth Resources Technology, lnc. and Marc Véronneau of Natural Resources Canada for ongoing collaboration and work towards defining a common model for North America.

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REFERENCES

TS04B -Heights, Geoid and Gravity, 5691 Daniel Roman and Neil Weston Beyond GEOIDI2: Implementing a New Vertical Datum for North America

FIG Working Week20l2 Knowingto manage the territory, protectthe environment, evaluate the cultural heritage Rome, Italy, 6-10 May 2012

Bur5a, M., Kenyon, S., Kouba, J., Sima, Z.,Vatrt, V., Vítek, V. and VojtÍ5ková, M. (2007) The geopotential value W 0 for specifying the relativistic atomic time scale and a global vertical reference system, J. Geodesy, 81 (2), 103-110, DOI: 10.1007/s00190-006-0091-3. Cheng, Y., and Andersen, O.B. (2011) Multimission empirical ocean tide modeling for shallow waters and polar seas, J. of Geophys. Res., 116, doi: 1 0. I 029 1201 1 JC007 172.

Drinkwater M.R., Haagmans R., Muzi D., Popescu A., Floberghagen R., Kern M., and Fehringer M. (2006) Proceedings of 3rd lnternational GOCE User Workshop, 6-8 November, 2006, Frascati, ltaly, ESA sP-627,2007.

Foreman, M. G. G., W. R. Crawford, J. Y. Cherniawsky, and J. Galbraith (2008), Dynamic ocean topography for the northeast Pacific and its continental margins, Geophys. Res. Leff., 35, L22606, doi: 1 0. 1 029/2008G1035 1 52.

GRAV-D Science Team (2011). "GRAV-D General Airborne Gravity Data User Manual." Theresa Diehl ed. Version 1. Available DATE. Online at

Pavlis, N.K., S.A. Holmes, S.C. Kenyon, and J.K. Factor, (2008) An Earth Gravitational Modelto Degree 2160: EGM2008, presented at the 2008 General Assembly of the European Geosciences Union, Vienna, Austria, April 13-18, 2008.

Roman, D.R., Wang, Y.M., Henning, W., and Hamilton, J. (2004) Assessment of the New National Geoid Height Model, GEO|DO3 , SaLlS, 64 (3): 153-162.

Roman, D.R., and Weston, N.D. (2011) OPUS-Database: Supplemental Data for Better Datum Conversion Models, presented at the F.l.G. Working Week in Marrakeck, Morocco, May 18-22,2011.

Smith D.A. and Milbert D.G. (1999) The GEO|D96 high-resolution geoid height model for the United States, J Geod. 73:219-236.

Snay, R.4., (2003) lntroducing Two Spatial Reference Frames for Regions of the Pacific Ocean, SaLlS, 63 (1),5-12.

Tapley, B. D., Bettadpur, S., Watkins, M., Reigber, C., (2004), The gravity recovery and climate experiment: Mission overview and early results, Geophys. Res. Lett., 31 (9), L09607, 1 0.1 029 t2004G10 I 9920.

Thompson, K. R., and E. Demirov (2006), Skewness of sea level variability of the world's oceans, J Geophys. Res., I I I, C05005, doi:10.1029/2004JC002839.

Wang Y.M., Saleh J., Li X., Roman D.R. (2011) The U.S. Gravimetric Geoid of 2009 (USGG2009): Model Development and Evaluation, J. Geod., 86(3), 165-180, DOI: 10.1007/s00190-01 1-0506-7.

Weston N.D., Soler T., Mader G.1., (2007) NOAA OPUS - Web-based Solution for GPS Data, GIM I nternational 21 (4):23-25.

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BIOGRAPHICAL NOTES

Daniel R. Roman, Ph.D., has been a Research Geodesistwith the National Geodetic Survey since 1999. He is the team lead for Geoid Modeling and Research as well as the Principal lnvestigator for the Gravity for Redefinition of the American Vertical Datum (GRAV-D) Project. He developed GEO|D99, GEO|DO3, GEOlD06, GEOlD09, and associated models.

Neil D. Weston, Ph.D. is the Chief of Spatial Reference System Division at the National Geodetic Survey and a principle behind the development of OPUS. He has also been involved in the development of OPUS-DB and OPUS Projects.

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Vertical Datum NEW MAPPING STUDIES CONVERT TO UPDATED VERTICAL DATUM

WHAT IS A VERTICAL DATUM? HOW DOES FEMA'S USE OF NAVD88 AFFECT YOU? A vertical datum is a base measurement WHO IS AFFECTED? point (or set of points) from which all The most frequent users of vertical datum elevations are determined. Without a include floodplain managers, surveyors, Property owners should not be common datum, surveyors would calculate engineers, builders, and insurance agents affected by a vertical datum and companies. Historically, the most different elevation values for the same change. lnsu¡ance rates (where location. Historically, that common set common vertical datum used by FEMA has elevation data are required and rates of points has been the National Geodetic been NGVD29. Many existing documents Vertical Datum of 1929 (NGVD29). (e.9., Flood lnsurance Rate lvlaps lFlRMsl, aren't grandfathered) and building However, as a result of advances in Elevation Certificates, Flood lnsurance codes will be based on the Base technology, an updated vertical datum Studies IFlssl) provide elevation values Flood Elevations (BFEs)* shown on was created and has been officially based on the old datum. adopted by the Federal Government as new Digital Flood lnsurance Rate When working with these documents, a new basis for measuring heights: the Maps (DFlRMs). However, users elevation values based on different North American Vertical Datum of 1988 vertical datums cannot be used together of elevation data from multiple (NAVD88). directly. All the information be¡ng used sources (e.9., a FIRM and Elevation (elevation WHY IS FEMA USING NAVD88? values on FlRMs, Elevation Certificate) must take care that the Certificates, other maps and documents) NAVD88 is more compatible with modern elevation values are based on the must be reviewed to ensure they are all surveying and mapping technologies based on the same datum, same vertical datum. lf they are like Global Positioning Systems (GPS). lt . not, the values must be converted also is more accurate than the previous Determ¡ne what datums are used on nat¡onal vertical datum, NGVD29, which the documents. to the same datum before they are no longer is supported by the Federal . lf the datums are the same, continue used. Failu¡e to do so can result in Government. to use the maps and other information improper design (e.9,, building at together. FEMA's lVap Modernization effort provides . the wrong elevation) or misrating an excellent opportunity to incorporate lf the datums are different, stoo and the insurance premium. These NAVD88 into flood hazard information. convert all the elevation numbers to users include floodplain managers, This change will support the accurate the same datum before using the measurement of elevation by FEMA and information. surveyors, builders, and insurance (NFIP) National Flood lnsurance Program Every user of elevation data on FEMA's agents. problems stakeholders, and will avoid the products needs to be aware of the datums of maintaining information based on an on which their elevation values are based, obsolete datum. differences in datums among the different

Base Flood Eleyat¡on - The water-surface elevat¡on resulting from a flood that has a l-percent chance of occurrlng ln any glven year.

FEMA Page 1 Vertical Datum NEW MAPPING STUDIES CONVERT TO UPDATED VERTICAL DATUM

data sources they are us¡ng, the required WHAT IS THE EFFECT OF THE datum conversion, and how to apply DATUM CHANGE ON FLOOD it. Particular care must be taken when HAZARD INFORMATION? comparing elevation data on a new FIRM The datum change does not change the panel using NAVD88 with data from a relationship of the ground heights to the previous FIRM panel that was produced water surface, lt does change the value using NGVD29. The user must be sure to assigned to those he¡ghts that are printed convert elevation values to one common on the maps and other documents or vertical datum. For example, insurance encoded in digital data. agents and companies must be especially careful about using elevations based on For example, the figure to the left shows similar vertical datums when using the a hypothetical build¡ng and nearby water NFIP's "grandfathering" rule for rating. surface. The LFE of the structure is 1l-i- They must avoid using a Base Flood feet measured using NGVD2g, but 110 Elevation (BFE) value from a FIRM based feet us¡ng NAVD88. Similarly, the BFE is on NGVD29 with a building's lowest floor 109 feet measured using NGVD2g, but elevation (LFE) from an 108 feet using NAVD88. The difference Elevations in Elevations in Elevation Certificate based between the elevations is the same with NGVD29 NAVD88 on NAVD88. The error could both datums: 2 feet. be significant if they are not first converted to the same This figure also illustrates two other points raised previously: '11't' 11o', vertical datum. Similarly, NGVD29 NAVD88 when calculating a new 1) The main effect of the datum (LFE) (LFE) premium with a BFE based change is a different value \ /. on NAVD88 and a bu¡lding's assigned to an elevation. For LFE based on NGVD29 example, in the figure, the from an older Elevation lowest floor of the same Cert¡ficate, the elevations structure is assigned one value

I 09' 1 08', should be converted to the when measured using NAVD88 NGVD29 NAVD88 same vertical datum. and a different value when (BFE) (BFE) measured using NGVD29. Flood Elevation Elevations in a local area all shift by the same amount, so the relative relationships are not changed.

Page2 FEMA FLEE D MAP MEDERNIZATIEN

Vertical Datum NEW MAPPING STUDIES CONVERT TO UPDATED VERTICAL DATUM

2l When comparing two values, STAY INFORMED ABOUT THE they have to be measured from CHANGE FURTHER INFORMATION the same datum. For example, Flood maps are changing, and so is the do not compare the LFE of the lf additional details are needed, the vertical datum being used. Floodplain structure measured using managers, surveyors, engineers, builders, following resources may be helpful. NAVD88 (l-l-0 feet) with insurance agents and companies, and a BFE measured using NGVD29 . For FEMA's guidelines regarding other users of elevation data from (109 feet). This would yield multiple sources (e.9., a FIRM and vertical datum conversions, visit: an incorrect difference in Elevation Certificate) must take care ljr.¡. elevations. Using this example, r.: ..!,,. ',, ,¡, .'¡¡r . I y.(' ( l,Í: that the elevation values they use are the difference would be !r,')t. . ()r and Click On Appendix B based on the same vertical datum. lf incorrectly calculated to be they are not the same, the values must . policy, presented 1 foot (110 - 109), compared to FEMA's NAVD88 be converted to the same datum before the correct elevation difference in Procedure Memorandum 41 (110 they are used. Failure to do so can of 2 feet - 108). (March 2006), can be found at: result in improper design (e.9., building WHAT INFORMATION DOES at the wrong elevation) or m¡srating the Li:( '\,!,J!,, 'Unt¡ -,{ì!, L, i[] trtO\,r,It./fliltì, FEMA PROVIDE ON CONVERTING insurance premium. The property owners' ntelto.l: ' ìtìltì BETWEEN VERTICAL DATUMS? risk is not affected by a vert¡cal datum . change because all elevations ¡n the local Additional information on vertical The difference between the two datums arca are changed by the same amount. datums, including the conversion varies from location to location. FEMA provides guidelines regarding where from NGVD29 to NAVD88 at any conversion factors (offset values) location, can be found by visiting should be calculated and the process the National Geodetic Survey site: for converting unrevised elevation data from old flood studies into new flood iltltr,'.ri',vv, iìlls,iì04.ì Eov studies. The exact conversion factors . For more information about flood will be listed in the accompanying FlS. insurance, visit: General conversion factors also may be shown on the FlRlVl panel. Where a county llrttiT wlvv riooosntíin gor boundary and a flooding source with . For additional details about unrevised NGVD29 flood elevations meet, an individual offset will be cafculated and FEMA's Map Modernization effort, applied during the creation of the new go to: hÌ1r,, /:vlv, ienra go,/' l)lal, DFIRI\4. ¡rreveni fhrr,nn rìailr ìl'ln

FEMA Pa¡le 3