How to Start
- Surface Difference option of Layer menu in Main Menu bar
What it Does
This dialog computes the difference between two selected surfaces and produces a difference surface object in the scene.
The Surface Difference wizard is used to initiate several kinds of surface-surface and surface-plane queries. Object types that can be queried are selectable through page 1 and page 2 of the wizard, and the type of query to be applied is selected via the query type radio buttons on page 3. The selected query is applied to the specified data objects, and the scope of the query is specified on page 4 of the wizard. By default, the query is applied to the common or intersection area of both input surfaces. However, a polygon can be used to constrain the query scope to within a certain area.
Whenever you perform surface difference operations the algorithm always works in the following manner.
- It determines which surface has the highest resolution (smallest bin size). If both have the same resolution than the first surface (Surface of Interest) is used as the highest.
- For each bin of the highest resolution surface the algorithm determines the height at the bin center from the other surface using a distance weighted average interpolation of the nearest bins at the point of interest. Data nulls (holes in the surface) are accounted for in this calculation.
- The difference is then determined from the two height values and written to the output file.
In doing the interpolation, if the interpolation point falls in a data holiday or outside the bounds of the second surface then no difference will be determined at that point. The resulting difference surface will have the same cell centers as the highest resolution surface but will be cropped as necessary to include only the surface area in common for both surfaces (and to account for any further constraints set on page 4 of the wizard). For example, if one surface is fully contained inside the other then the resulting difference surface will only cover that contained area.
Select the first surface for the difference operation. Click Next to proceed to the next page of the wizard or Cancel to cancel the operation.
Select the second surface for the difference operation. Click Next to proceed to the next page, Back to return to the previous page of the wizard or Cancel to cancel the operation.
Select which data to use for the difference operation. Click Next to proceed to the next page, Back to return to the previous page of the wizard or Cancel to cancel the operation.
Select the constraints of the difference operation. When loading constraints from a polygon file it is important to note that these files can only be ACSII files. Click Next to proceed to the next page, Back to return to the previous page of the wizard or Cancel to cancel the operation.
Review your selected options for the difference operation. Click Finish to proceed or Cancel cancel the operation. When complete, a Fledermaus static surface will be added to the project representing the results of the surface difference operation. You will also see the statistics for the surface difference.
Surface Statistics Summary Dialog
This dialog displays the resulting statistical information from the surface difference operation. You can save the statistics to a text file using Save Text As button.
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