MICRODEM Trend Surface

Option on

• Raster GIS menu, Geomorphometry analysis, with a DEM
• Stats button  table display window, with points upon a line to which you want to fit a plane

Display multiple windows with its results: Graph with trend surface versus DEM. A map window with deviations from trend surface (difference between DEM and trend surface). Modify this map like any other map. Histogram of deviations between DEM and trend surface. Text window with statistics. This will include the correlation coefficient r and r², the coefficients for the trend surface, and some data on the goodness of fit and the variation explained by the surface (see Davis, 1986, for an explanation of this table). Map window with trend surface. Modify this map like any other map.

This option fits a trend surface to the current DEM or database. You should be aware of the following limitations:

The entire DEM will be fit. If you do not want to use portions of the DEM, edit the DEM first.  To speed up the computations, the program will decimate the DEM to  use 250,000 points or fewer to compute the trend surface.

All variables are in meters, with the elevation (z) and the easting (x) and northing (y) UTM coordinates. To avoid math overflow problems, the x and y coordinates are shifted to place the origin at the SW corner of the data set. The shift will be indicated in the statistics window.

• The DEM must be capable of conversion to UTM coordinates, which will be the x and y coordinates used. The input DEMs in mind when the option was created were USGS 1:24K and 1:250K (DTED). 
• The z units should be meters to correspond with the UTM coordinates. 

Potential problems and their solutions:

1. The deviations from the trend surface mirror ridges and valleys in the valley. Consider editing the DEM to remove the deep stream valleys.

Last revision 9/15/2016