In Order to Do This Assessment Each Group Will Have to Do the Following

Accuracy Assessment - Field Validation

Where are we going, and what for?

Accuracy assessments determine the quality of the information derived from remotely sensed data (Congalton and Green, 1999). Assessments are a complex subject and a fairly immature one. Readers with specific interests in accuracy assessment are directed to Congalton and Green's book, Assessing the Accuracy of Remotely Sensed Data from which most of the concepts outlined on this page are taken.

Assessments can be either qualitative or quantitative. In qualitative assessments, we determine if a map "looks right" by comparing what we see in the imagery with what we see on the ground. This is not usually done in a rigorous way, but rather in a "quick and dirty" way. General accuracy is the goal in this case, and error and its sources are not as important. This is usually a first cut assessment. Quantitative assessments attempt to identify and measure remote sensing- based map error. In such assessments, we compare map data with reference or groundtruth data (where groundtruth data is assumed to be 100% correct).

Types of accuracy to consider

We can think of two types of accuracy. First, location accuracy. Are the objects that we map and the data sources themselves correctly located on the earth and to one another? Second, classification accuracy. Is a material on the earth or a condition mapped correctly, or has a material been misidentified and hence mapped incorrectly?

Error sources •Data Acquisition •Data Processing •Data Analysis •Data Conversion •Error Assessment •Final Product Presentation

Accuracy assessment sampling schemes

Before any remote sensing acquisition takes place, a sensible accuracy assessment should be planned. Depending on the questions being asked and the region being imaged, some types of assessment will be more feasible than others. Accuracy assessments in general are expensive and attention must be paid to limitations imposed by region, sensor, weather, etc. versus the statistical requirements imposed on the study.

Our assessment this week will be qualitative in nature. The objective is to get a “quick and dirty” assessment of the accuracy of our map and to help us related the map to the ground. In order to do this assessment each group will have to do the following:

1. Identify one ground-truthing site that you are confident was classified correctly. It should be a site you can find in the field using some discernible feature (river, road intersection, stand boundary, etc).

2. Identify two ground-truthing sites where the classification is in question.

3. For the three sites, determine the UTM Zone 18 coordinates (easting and northing) down to one decimal place. We’ll use UTM coordinates in the field because our image is stored in UTM, and UTM (meters) are easier to work with in the field than latitude and longitude. We can also set our GPS units to read UTM coordinates.

4. Record the information for each site in the online Google Docs Spreadsheet. The spreadsheet is found at:

https://spreadsheets.google.com/ccc? key=0AnjmROheiAPZdGJpMExlTjE4dmwzWkZyc1RtbkVqVHc&hl=en&authkey=CL 7Tp5gJ

The first three records (Group 99) are example records. Be sure to save your changes to the spreadsheet before you exit the Google Docs page.

5. Write down information about your sites for your own use in the field. Keep it with you and don’t loose it.

Here are some rules related to site identification:

a) Sites must be within 200 meters of a travelable road. (by a 15 passenger van)

b) Sites must be within the designated checking zone (DCZ.shp). This is the area we will visit on Wednesday.

c) Sites can be any size or shape. If the site is small, pick the center of the site to record its coordinates. If it’s a large site, pick an edge closest to the access point (ie. Road) and record the coordinates. d) If the site is NOT on public land, it must be visible from the road or a public right of way. e) The site must be in a safe place for us to park the van (rural road or some kind of pull off like a gas station) f) Avoid sites that require steep hikes up or down to access. g) Avoid sites where the land cover/land use cannot be determined due to snow cover. Some ag land uses can still be determined but it becomes much harder.

Here are the fields in the spreadsheet

Group_No: Your group number. Will be assigned in class. Don’t forget it!

Site_No: Site 1,2, or 3

UTM_east: UTM coordinates for the easting down to one decimal place.

UTM_north: UTM coordinates for the northing down to one decimal place.

Status: Possible values are “Confident”, “Unsure”, “Unknown”

Comments: Any comments you want. Keep them short.

There are four layer files you will need for reference. They are located in the K:\gg1002\Data\Landsat\Middlebury_area\field_checking folder.

The layers are:

Conserved Lands.lyr

Roads.lyr

Town Boundaries.lyr

DCZ.lyr