MacDEM User's Guide

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Undo

Restores the DEM to the way it was before the most recent change to the elevation data. If there is not enough memory to create an Undo buffer, you will be warned before changes are made to the DEM. You can then continue without Undo, or cancel the change. Closing other windows or applications may free enough memory that Undo becomes available again.

Copy

Copies the contents of the Header Info window to the clipboard. Available only when the Header Info window is open for a DEM.

Merge Current DEMs

Merges all open DEMs into a single DEM.

To preserve elevation resolution, MacDEM will use the finest elevation resolution of all the DEMs which still supports the range of elevations of all the DEMs. If necessary MacDEM will normalize the merged DEM's elevations.

The DEMs must have compatible horizontal scaling to be merged. Specifically, whatever values the front window's DEM has for the following parameters, all the other DEMs must have the same values:

Pattern
Ref. System
Zone
Units (plan)
Resolution(e) (note: e=east, n=north)
Resolution(n)

These values can be viewed in the Info window. If a DEM does not have the same values as the front window for any of these parameters, you will see this error message:
"DEM incompatible with front window".

If a DEM has different values for Resolution(e) and (n) than the other DEMs, you may be able to use Resample to convert its resolution so it is compatible, if the difference is an integer scale factor (see below). If any of the other parameters don't match, then the DEM can't be merged using MacDEM.

Example of using Resample to make DEMs compatible for merging. To merge a 10m DEM with a 30m DEM:

Set the 10m DEM's zoom to 1/3x.
Select Resample in the Edit menu. The horizontal resolution is now 30m, matching the other DEM.
Merge

You could instead set the 30m DEM's zoom to 3x and resample it, converting it to a 10m DEM, and then merge. This will preserve the higher resolution of the 10m DEM, but may also cause edge mismatches between the DEMs to be more noticeable.

MacDEM currently does not perform any datum transformations when merging DEMs. This can cause the DEMs to not match exactly at the edges, leaving gaps. Use the Fill Gaps filter or the Edit Tool to fill these in.

Note that there are also various error sources in DEM data which can prevent the edges of merged DEMs from matching up exactly.

Set DEM Origin...

Displays a dialog box allowing you to set or change the coordinates of the front window's DEM origin. Primarily used for georeferencing a Raw Data file that has been imported, but can also be used to adjust any DEM's origin. This might be useful for shifting DEMs to improve the results of a Merge.

Resample

Resamples the current DEM to a finer or coarser resolution to match the current zoom. If the current zoom is 1/2x or less, the DEM is subsampled to a coarser resolution. If the current zoom is 2x or greater, the DEM is supersampled using bilinear interpolation. Supersampling will tend to generate faceted terrain--the facets can be smoothed using the Mean Filter function.

The Resample menu item is disabled if the zoom is 1x since resampling at 1x wouldn't do anything.

DEMs should be merged before supersampling if possible to reduce the likelihood of artifacts and gaps at seams between DEMs.

Edit Tool

Enables and disables the Edit Tool for the current DEM. When enabled, the Edit Tool lets you "draw" the elevation box value into the DEM by clicking or dragging in the DEM. It is easier to see what you are doing if you zoom in a little. It also sometimes helps to turn lighting off. You can temporarily disable the Edit Tool to pick up a value by holding down the option key.

The Edit Tool can also be chosen via the tool bar.

Selection Tool

Enables and disables the Selection Tool for the current DEM. When enabled, the Selection Tool lets you drag-select a rectangular region of the DEM. The selected area includes pixels under the rectangle's outline. After making a selection, the selection can be resized by grabbing the edges and dragging.

The Selection Tool can also be chosen via the tool bar.

Crop

Trims the current DEM to include only the area you have selected using the Selection Tool. The DEM's georeferencing data is updated accordingly. Crop is disabled if there is no selected area or if the selected area is too small.

Edit menu