Monday, March 21, 2011

Comparing Lithologic Contacts

One important thing that I have learned over the last two quarters is that geologists will rarely map contacts exactly the same.  To make things worse, if you can map contacts using remote sensing methods, the contacts may also be quite different.  Sometimes contacts are obvious since they follow specific topographic features.  Other times, they're difficult to track with only float giving some indication of presence.  Because of the variability in mapping, some geologists may want to easily compare their contacts with previous efforts, other geologists, or remotely mapped contacts.  Here is one way to quantify differences between contacts.

This method assumes that you have already input two sets of contacts in the field area into a GIS as polylines.  This method should work on any GIS, including R, but some steps are specific to Arc.  In this example, Contact A and Contact B are a set of contacts for the same related lithological area.  They are being compared using the shortest distance between contacts.

The euclidian distance raster is the background.  Central to the screen shot are the two sets of contacts being compared.  The black is Contact A, the blue is Contact B.  The point features, used for sampling the euclidian raster, should make it clear which contacts are being compared.  The points have an interval of 20 m.
  1. Select only Contact A
  2. Generate a Euclidian Distance raster for Contact A.  Be thoughtful about your cell size (I have used 0.5 m) and extent of the new raster.
  3. If using Arc, enter Editor mode and create a point layer.  Ensure that point layer and the contact layer are in editing mode.
  4. Select only Contact B, merge the contact lines (perhaps in a new feature class).
  5. Generate an interval, your choice, of points that coincide with the merged Contact B.  In ArcMap 9.3., you can use the "Divide" tool under the Editor menu (... maybe).  In ArcMap 10,you can use the "Construct Points" tool under the Editor menu. Save and stop editing.
  6. In ArcMap, use Spatial Analyst's Sample tool.  Use the Euclidian distance raster as your input raster and your line coinciding points as your point features ("Input Location Raster or Point Features").  Define your output table's location.  The default resampling technique is fine.  See above figure.
  7. Press OK and you will now have a table with the shortest distance from the points along Contact B to the lines of Contact A
  8. Now import the distance data into your favorite statistics software and see how your data compares (or just use Arc's built in statistics tools).
Output table with sampled distances between Contact A and Contact B.

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