GMDE Step-By-Step.Pages

GMDE Step-byStep Guide Accomplishing Tasks in GMDE: a step-by-step guide GMDE is a powerful program for extracting quantitative data from a geologic map or satellite image. This document provides a step by step approach to accomplishing common tasks in the program as a supplement to the users manual. There are many detailed adjustments, not covered here but reviewed in the Users manual, that one can make via the Inspector palette. Most important is perhaps the choice of elevation server that GMDE uses to get elevations of points. The default is the Map- Quest server which makes SRTM-based topo data available globally. If you are working with maps from the United States, you might want to switch to the USGS elevation server which uses the national elevation database and tends to be closer to the values you see on topo maps. Contents Georeference a Map 2 Set the Coordinate System Offset 3 Digitize Existing Strike and Dip Symbols 4 Calculate Orientations from 3-Point Problems 5 Select a Subset of Strikes and Dips for Analysis Using Lasso 7 Measure Stratigraphic Map Thicknesses from 3-Point Problem 8 Measure Stratigraphic Map Thicknesses using mapped strike and dip 9 Propagate Errors in Your Analysis 10 Digitize Contacts 11 Construct a Down-Plunge Fold Profile 12 Get the Elevation of a Cylindrically Folded Surface at Depth 14 Construct Topographic Profiles with Contacts and Apparent Dips 15 Plot Topo Profile Along Part of a Line of Section, only 18 Calculate Piercing Points on a Fault or other Planar Surface 18 R. W. Allmendinger © 2016 !1 GMDE Step-byStep Guide Georeference a Map Before a map can be used for quantitative calculations, you need to georeference the map. This will set the scale and enable the program to get the elevation of any clicked point automatically from the Internet. If you georeference a map, you do not have to set the scale separately! Once you have opened a scanned map, before doing anything else, proceed as follows. 1. Select Settings>Georeference (⌘G). This dialog box (to the right) will lead you through the entering of the four corners of the map, starting with the upper left corner. 2. In the map pane, click carefully on the coordinates at the upper left corner of the map. GMDE will automatically select the box where you will type in the longitude of the corner point. Then, you just need to tab to the latitude box. Note that west longitudes and south lati- tudes are entered with a negative number. All values are entered in decimal degrees. 3. Once you’ve click on the upper left corner of the map and entered the coordinates, GMDE is ready and waiting for you to click on the upper right corner. Once you do so, text boxes will appear where you can enter the longitude and latitude as before. GMDE attempts to save you some typing and assumes that the latitude will be the same as that previously entered, but you can change that to the actual value if they are different. 4. Proceed as above with the lower right corner and then the lower left corner. If you are georeferencing a standard map, GMDE will have correctly filled in the coordinates completely for the lower right corner 5. When all four points have been entered, click okay. GMDE will then ask you what units you want to use for the map. The georeferencing procedure generally works with maps and images in a variety of different projections because they are “close enough” to a UTM projection. In some cases where the map or satellite image is too distorted or in a geographic projection, GMDE will inform you that the least squares best fit of the map to a UTM projection is poor and to proceed with caution. In that R. W. Allmendinger © 2016 !2 GMDE Step-byStep Guide case, you may want to use a different software package to reproject the map into a UTM projection to start with. Set the Coordinate System Offset If you plan on using GMDE’s ability to download elevations for clicked points from elevation servers on the Internet, it is especially important that you set the coordinate system offset. This offset is the difference between whatever datum was used for the map (and commonly you will not know because it is not listed on the map) and a WGS84 datum, which is what elevation servers use. GMDE can calculate a uniform offset for the entire map area by comparing the coordinates of a point in Google Earth with the same point on the scanned map you are working with. Proceed as follows: 1. Locate a point that can be easily and accurately identified both in Google Earth and on your map. Usually, this will be a road intersection, the intersection of two creeks or maybe a building or well defined peak, meander bend, etc. 2. In Google Earth, determine the longitude and latitude of the feature and write them down on a piece of paper. Six significant figures is good! It helps to zoom in a lot (as shown in the above example) to get the most accurate reading. R. W. Allmendinger © 2016 !3 GMDE Step-byStep Guide 3. Return to GMDE and choose Settings>Set Coordinate System Offset. A dialog box will appear instructing you to click on a very well defined point, that is the same one that you got the coordinates for in Google Earth. 4. Click on that point and then in the same dialog box, two text boxes will appear where you can enter the same longitude and latitude that you wrote down in Google Earth. Click Okay. 5. A second dialog box will appear telling you the exact offsets that GMDE calculated for your map, usually on the order of hundreds of feet or tens of meters. Once you have set the coordinate system offset, if you hover the mouse over a corner of the map that you entered in the georeferencing part, the latitude and longitude that appear in the location box in the lower left corner of the map will no longer be what you entered but some- thing slightly different! Don't worry! GMDE is now showing you the coordinates of the point in WGS84. GMDEs ad- justment is approximate because it is constant over the entire map. None- theless it should be close enough assum- ing your map area isn’t too large. At any time, you can see the metadata for your map that GMDE has calculated by selecting Settings>Show Map Metada- ta. An example is shown to the right. Digitize Existing Strike and Dip Symbols GMDE makes it (relatively) easy to extract orientation data from a scanned map. To do so, proceed as follows: 1. Select Settings>Mode>Strike & Dip Only. Make sure that “user entered strike and dip checkbox is checked in the tab panel on the right 2. To capture a strike line, click the “click” button to the left of the word “Strike” or select Operations>Drag Strike Line (⌘G). 3. Click the mouse at the center point of the scanned strike line (which is usually where it intersects the dip tick mark) and drag the mouse in the direction of the right hand rule strike. The line will expand out symmetrically in both directions from the center R. W. Allmendinger © 2016 !4 GMDE Step-byStep Guide point. You can drag as far as you want and, in fact, the farther you drag, the more accurate your measurement will be. 4. GMDE will display a dip tick mark oriented correctly (i.e., using the right hand rule) for the direction in which you are dragging. It will also display an interactive dip value that will increase in number the longer you make the strike line (see diagram, below). 5. Once you release the mouse button, GMDE will automatically record both the strike and the coordinates of the measurement in the panel to the right, as shown above. 6. GMDE will also paste the interactive dip from step 4 in the dip text box. The contents of that text box will be selected so that you can type in the dip value if the interactive- ly determined dip was not correct. 7. If you did not drag in the direction of the right hand rule strike, you can correct it by selection the quadrant direction of the dip from the popup menu just above the dip text box. The interactive display of the dip tick mark makes this unlikely. 8. When your are satisfied with the measurement, click the “Record S & D” button. Your measurement is not saved until you do this step! Calculate Orientations from 3-Point Problems The orientation of any planar feature can be determined from any three points on its surface so long as at least one of the three points is at a different elevation than the others. GMDE makes this task especially easy by allowing you simply to click on the three points. Proceed as follows: R. W. Allmendinger © 2016 !5 GMDE Step-byStep Guide 1. Locate a stratigraphic contact or other planar feature where it crosses topography by “V-ing” across ridges and valleys. Make sure that the “user entered strike and dip” checkbox is NOT checked. 2. To click on the first point, there are two different methods: • More straightforward: Press the “click” button located next to the top row of coordinates in the “Surface 1 3 Pt Calculation” group box and then click the mouse on the geological contact trace on the map.

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