GENERATING GEOLOGIC CROSS SECTIONS WITHAUTOCAD MAP AND ARCVIEW GISPage 1 of 14IntroductionGenerating geologic cross sections with AutoCAD Map and ArcView GIS requires the use of 3Dgeometry commands within AutoCAD to produce the proper perspective view of the topographicprofile and geologic subsurface structures. The following discussion outlines this process step-by-step. The assumption is made that the geologic map and cross section line (A-A’) have alreadybeen prepared, and that a DRG quadrangle is loaded for topographic control.STEP 1: Insert topographic block markers in base mapThe first step consists of designing a topographic marker block that will be inserted where ever acontour line crosses the cross section reference line (A-A’). The block should be designed to havean easily identified center point, and it should be visible from any 3D perspective. For example, agood design would be 3 mutually perpendicular lines intersecting at midpoints (i.e. “Jumpingjacks”). In this example the block will be named “3DCROSS”. Zoom into the drawing in theregion on the “A” side of the cross section line. Where the first contour line crosses the A-A’ line,use the insert command to place the “3DCROSS” block exactly at the point of intersection. To dothis use the “.xy” coordinate override. Also, indicate from the insert dialog that you will specifythe insertion point from the command line. Figure 1 displays the correct setup for the insertdialog. Note that the scale factor is set to 20 meters, appropriate for the 1:12,000 RF in thisexample. When the OK button is clicked the user will be prompted for the insertion point of theblock:Specify insertion point or [Scale/X/Y/Z/Rotate/PScale/PX/PY/PZ/PRotate]: .xyRespond with the “.xy” coordinate dot command. This allows you to specify the x and ycoordinates of the block with the pointing device, but then set the z coordinate with an elevationtyped in at the command line. Use the “nearest” object snap to set a point on the cross section linewhere a topographic contour crosses the line. You will then be prompted for the Z coordinatevalue:(need Z): 7200This procedure will put the block marker at the correct elevation and exactly on the A-A’ crosssection line. Continue until all topographic contours are accounted for along the A-A’ line. Whenall of the contour line intersections are accounted for, the block markers will form the topographicprofile along A-A’ even though you cannot see the profile because your viewpoint is parallel tothe plane of the cross section.GENERATING GEOLOGIC CROSS SECTIONS WITHAUTOCAD MAP AND ARCVIEW GISPage 2 of 14In addition to contour line intersections, you may also wish to mark where geologic contactscross the A-A’ line. These markers should be a different color than the topographic markers.Figure 2 displays an example base map with inserted block markers on contour line intersectionsand on geologic contact intersections.STEP 2: Export topographic block markers to an external fileThe next step will export the topographic profile block markers to an external AutoCAD file. Thisfile will eventually contain the geologic cross section. Start the process by typing the “WBLOCK”command at the command prompt. Figure 3 displays the dialog window generated by thiscommand. You should select just the topographic control blocks and the A-A’ cross section lineas objects for the WBLOCK command. In this example the objects are exported to the externalfile “BoxCanyon_CrossSection.dwg”, which will become the geologic cross-section. Make surethat you retain the original objects before selecting the OK button. When the OK button isselected, the information will be written to the indicated external file. You can know quit the basemap and load the external file just created. STEP 3: Setting up the UCS coordinate systemAll operations completed so far in AutoCAD have been in the default coordinate system namedthe “world” coordinate system (WCS). You may have notices the coordinate system orientationicon in the lower right portion of the AutoCAD window indicating that positive X increases to theright, positive Y upwards. When the topographic profile marker blocks were exported to anexternal file the WCS is unchanged. Verify this fact by loading the external file into AutoCAD atthis time. At this point you need to define a user coordinate system (UCS) that makes the current Z axis(i.e. elevation axis) the new Y axis. Another way of visualizing this would be to rotate the currentWCS about the X axis by 90 degrees. In effect, this will correctly orient the A-A’ cross section sothat we are viewing the vertical plane passing through A-A’. Before we can define the new UCSof the cross section we need two lines originating from the “A” point, one being the X axis of thenew UCS, the other being the Y axis. The already existing A-A’ line will serve as the X axis, butwe need to construct the Y axis line of the new UCS. To do this correctly we need to view theentire drawing from a 3D perspective. Select the menu sequence “View > 3D view > Vpoint”.You will then see a dynamic “bullseye” type graphic. Move the pointer to the interior circle at thesoutheast quadrant and then click the left mouse button. You should then see something similar toFigure 4. Note that the marker blocks are too far above the A-A’ cross section line. This reasonfor this is that the X and Y coordinate system is UTM, which uses meters for units, but theelevation contours are in feet above M.S.L. The inconsistency in units will be corrected in a laterGENERATING GEOLOGIC CROSS SECTIONS WITHAUTOCAD MAP AND ARCVIEW GISPage 3 of 14step.Next we need to draw a 3D polyline to define the orientation of the new UCS Y axis. Zoom in onthe “A” end of the cross section line, and then used the “Draw > 3D polyline” command. For thestart point, use “endpoint” snap to snap to the end of the existing cross section line, and then forthe second vertex type:Specify endpoint of line or [Undo]: @0,0,1500This sets the endpoint directly above the “A” end of the cross section line at an elevation of 1500units. The value 1500 was chosen because it exceeds by a small proportion the total topographicrelief of this specific cross section line. You should make this value appropriate for your situation. Now the new UCS can be defined. Type the command “UCS” and hit the “enter” key. Use thefollowing responses (bold