ArcGIS Outcrop Prediction Example Problem

IntroductionThis document discuss the step-by-step example of using ArcGIS with 3D and Spatial Analyst extensions to process a outcrop prediction geologic map starting with a digital elevation raster and the orientation of the bottom and top contacts of a inclined tabular geologic unit. A Python application will create the planar elevation rasters based on the attitude of the top contact and a single position on the map where the attitude was observed. The bottom contact raster is derived from a knowledge of the thickness of the unit.

Starting Data

You will be able to download the below files from the GIT461 online site:

1. OPexample_start.mdb2. PlaneTrendSurf.py

If you are not enrolled in GIT461 download the starting data at the below web site:

http://www.usouthal.edu/geography/allison/gy461/gy461_project_resources.htm

Look for the section titled “Outcrop Prediction”, and then download from the links:

3. OPexample_start.mdb4. PlaneTrendSurf.py

Create a folder on a disk and save these files to that working folder. It is advisable to put any files created during this exercise in that same folder.

Open ArcMap and create a new project file (“OPexample.mxd”). With ArcCatalog make a copy of “OPexample_start.mdb” to “OPexample.mdb”, and then add all of the features from “OPexample.mdb” to the new ArcMap project. You should have something similar to Figure 1 when done. Note that in the figure the attribute table for the “bedding” feature is open, displaying the attitude of top contact outcrop (000, 40E). A geologic bedding symbol has been defined for this entity – your symbol will probably be a simple “dot” by default. You can display the coordinates of the data point be left-clicking on it with the “Identity” tool. The outcrop falls on the 110 contour so assume that value for the elevation of the top contact outcrop.

Generating the Elevation GridThe initial step in processing the outcrop prediction with ArcGIS is to generate an elevation raster grid from the survey data. The data in the feature class “Survey Point” has 3D properties so each point has an

attached “Z” elevation value. This means that you can use the “Spatial Analyst Tools > Interpolate > Spline” to generate an elevation raster grid from the survey points. Figure 2 displays the settings for this tool. Figure 3 displays the resulting elevation grid.

Generating the Top and Bottom Contact Elevation GridsThe python program “PlaneTrendSurf.py” will be used to generate the planar top and bottom contact grids. The input file for the program is described below:

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Line 1. trend and plunge degrees of the true dip vector (trend in azimuth format) (use “90.0, 40.0”).

Line 2. Elevation (Z value) of the measured attitude. (use “110.0”).

Line 3. "X,Y" coordinates of the position of the attitude measurement (use "500165.0, 3000085.0").

Line 4. SW corner (lower left) "X,Y" coordinates of the calculated raster image. (use “500000.0, 3000000.0")

Line 5. "Columns,Rows" in calulated raster (use "195, 145").

Line 6. Grid spacing between rows and columns of raster grid (use "1.0").

File name: ProbEx_top_in.txt

90.0, 40.0

110.0

500165.0, 3000085.0

500000.0, 3000000.0

195, 145

1.0

Output: ESRI raster file with following structure:

Line 1: "ncols=" {number of columns in raster grid}

Line 2: "nrows=" {number of rows in raster grid}

Line 3: "xllcorner=" {x coordinate of the lower left (SW) corner of the grid}

Line 4: "yllcorner=" {y coordinate of the lower left (SW) corner of the grid}

Line 5: "cellsize=" {cell size spacing between columns and rows}

Line 6: "nodata value=" {value that represents no data at grid node}

Line 7: 1st z value

Line 8: 2nd z value

Line (ncols * nrows + 6) : (ncols * nrows) z value

File name: ProbEx_top_grd.txt

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You can use “Notepad” or any similar text editor to create the input file. Just remember that comas are used to separate two values that occur on the same line, as in line 3 that contains the x and y coordinates of the outcrop point. This means that you cannot use comas within the x or y coordinate number.

The input file for the bottom contact – “ProbEx_bot_in.txt” in this example – will be identical to the top contact input file except that the elevation will be a lower value based on the thickness of the tabular unit:

d = t / cos(δ)

where d represents the decrease in elevation, t is the thickness of the tabular unit, and δ is the dip angle. For this example the thickness is 12.75m therefore:

d = 12.75 / cos(40) = 16.6m

The “ProbEx_bot_in.txt” file should appear as below:

90.0, 40.0

93.4

500165.0, 3000085.0

500000.0, 3000000.0

195, 145

1.0

Note that the only difference between the input files is the adjusted elevation on line 2. Proceed to create the ASCII elevation rasters “ProbEx_top_grd.txt” and “ProbEx_bot_grd.txt” from the above input files.

When the ASCII output grid files are created, use the toolbox tool “Conversion > To Raster > ASCII to Raster” to import the ASCII rasters to “Top_grd” and “Bot_grd” respectively. Both of the grids will appear very similar because both the top and bottom contacts have the same planar geometry- they are just offset in elevation by approximately 17m.

Create the Residuals by Subtracting the Elevation from Top and Bottom GridsResiduals are calculated by subtracting the elevation grid from the top or bottom grid to produce “top_resid” and “bot_resid” raster grid feature classes respectively. Both of these rasters have the following characteristics:

Grid Node Geometry

> 0.0 Contact is above topographic surface therefore the strata below the contact is exposed.

= 0.0 Contact outcrops at this point.

< 0.0 Contact is below topographic surface therefore the strata above the contact is exposed.

Note that if you symbolized all values < 0.0 with a blue color, and values > 0.0 with red, and 0.0 pixels black you would have a basic geologic map with blue representing the strata above the contact outcropping at the surface, red would be the outcrop of the strata below the contact, and the black pixels would trace the contact itself.

At this time run the toolbox tool “3D Analyst > Raster Math > Minus” to subtract the “ElevGrd” from the “Top_grd”. Note that in the “Minus” dialog window the 2nd raster name listed is subtracted from the 1st (top-most) raster name. Do the same with the “Bot_grd” (displayed in Figure 6) to produce both of the residual rasters “top_resid” and “bot_resid”.

Figure 7 displays the top contact grid residual “top_resid” in a gray-scale color scheme.

Reclassify the ResidualsThe residual rasters will be reclassified so that values <= 0.0 are assigned a value of 1, and values > 0.0 are assigned a value of 2. After this step both “top_reclass” and “bot_reclass” will have the following characteristics:

1Grid Node Value Reclassified Value

< 0.0 1 (strata above contact exposed)

>= 0.0 2 (strata below contact exposed)

The “3D Analyst Tools> Reclass > Reclassify” tool will be used to reclassify the two residual rasters as in Figure 8. In the Figure 7 initial window click on the “Classify” button to change the classification criteria. Figure 9 contains the settings in the second window – note that two classification levels are set with a break value at “0”. Use these same settings for the “bot_reclass” raster. Figure 10 shows the results of the reclassify operation on the top contact residual (“top_reclass”).

Add the Two Reclass RastersThis simplification will allow the top and bottom rasters to be “added” together to produce a raster that has the below characteristics:

1 Top Grid + Bottom Grid = Composite Grid

1 1 2=younger strata exposed

(strata above top and bottom contact)

2 1 3=intermediate strata exposed

(strata below top contact, above bottom contact)

2 2 4=older strata exposed

(strata below top and bottom contact)

Note that the condition of the top grid node = 1 and the bottom grid = 2 at the same map position is not possible if the top contact is structurally above the bottom contact. The composite grid separates the older, intermediate, and upper stratigraphic units into 3 unique values, therefore, it represents the geologic map in raster form.

Figure 10 displays the “3D Raster > Raster Math > Plus” dialog where the “top_reclass” and “bot_reclass” are added together to produce “reclass_comp”. Figure 11 displays “reclass_comp” composite as the raster geologic map. Note that the lower value pixels are stratigraphically younger therefore the colors in the table of contents window are in proper stratigraphic order.

Converting the Raster Geologic Map to Lithologic PolygonAlthough the “reclass_comp” is a raster geologic map there are several reasons why you may want to convert the raster into a polygon feature class. For example, if converted to a polygon feature the area of each polygon entity is automatically calculated whereas a fairly complex analysis would have to be processed to yield the same value with a raster. The boundaries between polygons are smoother and give a better quality to plotted maps.

The toolbox tool “Raster to Polygon” may be used to convert a simplified raster like “reclass_comp” to geologic polygons. Instead of trying to find it in the toolbox, try using the “Windows > Search” and type “Raster to polygon” into the search criteria text box. Click on the “search” icon to bring up the results in Figure 12. Click on the “Raster to Polygon” link in the search window results to start the tool. Fill in the tool window as indicated in Figure 12. Note that the results of the tool “Lithology” is sent to the project

geodatabase file, not a shape file. Make sure to do this so that you have access to the “Shape_Area” field for later calculations.

Figure 13 displays the “Lithology” polygon features categorized by “Gridcode” value. The value “2” is the younger stratigraphic unit, “3” the intermediate, and “4” the older unit. Because the symbology is normally sorted in descending order the symbology will be in stratigraphic order. In Figure 13 the “Identity” tool was used to click on the large older strata polygon – the results are in the small window. Note that the “Shape_Area” units are square meters.

Composing the Final Map ProductArcMap can easily add the final touches to the geologic map. First, use the “File > Page and Print Setup” to select an output printer and page size/orientation – use letter/landscape as indicated in Figure 14. Next, select “View > Layout” to enter into “layout” mode. Set the scale in the “Map Scale” edit box to 1:1000. Then use the “select” tool to re-size the frame surrounding the map to the dimensions and placement similar to Figure 15. Proceed to use the “Insert” menu to add the following items to the map:

1. North Arrow2. Scale Bar3. Title4. Legend

You should wind up with something similar to Figure 15.

CalculationsWith the lithologic polygon feature you can do a variety of simple calculations:

Problem 1: What is the map area of the intermediate stratigraphic unit? 5813.4 m2

Solution: Use the “Identify” tool to click on the intermediate unit polygon. When the results window is displayed look for the value of “Shape_area”. In this example the value is 5813.4 m2. The area units will always be the square of the unit used by the map coordinate system – in this case UTM that uses meters. ArcGIS automatically updates the “Shape_area” field when polygons are created or modified. The results window is displayed in Figure 16.

Problem 2: Assuming the intermediate unit is composed of coal what volume in m3 could be extracted from the map area? 100,430 m3

Solution Step 1: Use the toolbox tool “Raster to TIN” to generate TIN versions from the “ElevGrd”, “Top_Grd”, and “Bot_Grd” to create “Elev_TIN”, “Top_TIN”, and “Bot_TIN” respectively. When generating each of the TIN rasters use a value of “1.0” for the “Raster Tolerance”. Using a default value generally generates a TIN that is over-simplified.

Solution Step 2: Use the “Surface Difference” tool to calculate the following:

Vol1 = Elev_TIN – Bot_TIN {calculates the volume between the surface and the bottom contact}

Vol2 = Elev_TIN – Top_TIN {calculates the volume between the surface and the top contact}

Note that in both of the above that the elevation TIN is the reference surface. Figure 17 displays the “Vol1” calculation with the “Surface Difference” tool window. When the tool is complete a polygon feature is added to the project that will consist of an area east and west of the contact used in the tool. Because of the dip direction (east) the polygon to the east of the contact will contain the volume between the surface and the contact in the subsurface. The “Identify” tool can be used to access the volume amount.

Solution Step 3: Calculate the volume of the intermediate coal unit with the calculation:

Coal volume = Vol1 – Vol2 = 134567.8 - 34137.8 = 100,430 m3.

Note that this works because “Vol1” represents the entire volume of material from the surface down to the bottom of the coal unit. “Vol2” represents the entire volume from the surface down to the top of the coal unit. The “Vol1” – “Vol2” difference must then be the volume of the coal unit.

The “Vol1” and “Vol2” values above were determined by using the “Identify” tool on the east polygon in each case to retrieve the volume attribute. If you needed to calculate metric tons you would simply multiply the cubic meters by the density. For example if the density of the coal were 1.93 g/cm3 the tonnage of the unit would be:

Metric tons of coal = 100430m3 * 1.93g/cm3 = 193,830 metric tons

Problem 3: If the younger stratigraphic unit consisted of sand and gravel how much material (m3) could be mined from the mine area indicated in Figure 18? 9076 m3

Solution Step 1: Add the mine area polygon to the project from the starting “OPexample_start.mdb” file. At this point the “ElevGrd” and “Top_grd” raster data needs to be extracted for just the mine area with “Extract by Mask” tool:

Raster “Extract by Mask” Results

“ElevGrd” mask=”MineArea” “Elev_mine”

“Top_grd” mask=”MineArea” “Top_mine”

Solution Step 2: Use raster math tool “Minus” to subtract the elevation of the top contact from the surface elevation to yield a raster of vertical depth to top contact values:

1st Raster 2nd Raster Raster Math Tool Results

“Elev_mine” “Top_mine” “Minus” “mine_depth”

Solution Step 3: Each pixel value in “Mine_depth” represents the vertical depth from the ground surface down to the top contact of the coal bed, therefore, if you could determine the map area of each pixel multiplying the “Mine_depth” raster by this area constant value would yield another raster that represents the volume of the younger unit on a per pixel basis. This information is available by right-clicking on the “Mine_depth” raster name, and then selecting “Properties > Source”, and then looking for the “Cell size” attribute. In this case it is 1 meter x 1 meter so it is pointless to multiply by 1 m2. However, most rasters do not have 1 x 1 unit pixel size so here we are skipping a step that is normally required. For example, if the pixel area had been 2.5 m x 2.5 m = 6.25 m2 then you would use “Raster Math > Times” to multiply the “Mine_depth” raster by 6.25 to produce a new raster where each pixel would represent the volume of the mine under each pixel.

Solution Step 4: Because we can assume the pixels in “Mine_depth” also represent m3 volume we really just need a method to sum the value of the pixels in “Mine_depth” for the answer in cubic meters. You can do this with the symbology controls: right-click on “Mine_depth” in the table of contents window and select “Properties > Symbology”. Select “Categories” instead of “Stretched” for the classification type. Then click on the “Classify” button to display the Figure 19 dialog. The sum of the pixel values will be listed in the upper right “Classification Statistics” section. In this case the sum = 9076 m3.

Figure 1: Appearance of the starting data in ArcMap.

Figure 2: Interpolation of survey data into an elevation grid.

Figure 3: Elevation grid interpolated with the "Spline" tool from survey points.

Figure 4: Creating the top contact grid with "ASCII to Raster".

Figure 5: Appearance of the bottom contact grid after importing the ASCII grid.

Figure 6: Using "Raster Minus" to produce the bottom residual.

Figure 7: Reclassifying the top contact residual - initial window.

Figure 8: Settings the classification levels to create the "top_reclass" raster.

Figure 9: Appearance of the reclassified top contact raster.

Figure 10: Using the raster "Plus" tool to add the "top_reclass" and "bot_reclass" rasters to produce "comp_reclass".

Figure 11: Appearance of the composite reclass raster after adding the top and bottom contact reclass rasters.

Figure 12: The "Raster to Polygon" tool that converts the "reclass_comp" to a lithologic polygon feature.

Figure 13: Lithologic polygons symbolized in stratigraphic order.

Figure 14: Page and Print Setup menu option dialog window.

Figure 15: Map layout for example outcrop prediction problem.

Figure 16: Results of the "Identify" tool click on the intermediate stratigraphic unit.

Figure 17: Settings for the "Surface Difference" tool when calculating the volume between the surface and bottom contact.

Figure 18: Mine area shown in example problem map area.

Figure 19: Retrieving the raster statistics from symbology.