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Setting a project datum

The magnitude of coordinates for a particular coordinate reference system can have an impact on the accuracy of certain calculations in Muk3D. For local CRS’ where the magnitude of coordinates are close to zero or less than hundreds of thousands, its most likely not an issue. When working with UTM coordinates where the magnitude of the x coordinates may be hundreds of thousands, and the y coordinates are in the millions, there can be some unique issues that arise.

The image above is in UTM coordinates, and the surface was contoured at a very small interval. Some of the contours can be seen to be overlapping, which is a result on some numerical errors when displaying large magnitude values.

Another symptom may be unexpected deformation of lines or point markers, as shown below.

To limit this, we set a datum specific to the project extent that Muk3D can use for all modelling calculations. This project datum is usually the bottom left of the loaded geometry extents, and eliminates the need to consider the redundant distance from actual coordinate zero in each calculation involving each point. The project datum does not change the location of geometry in space (see Show project datum below), nor effect the coordinate system when exported. It is used only by Muk3D internally, but is never the less extremely important to achieve the intended accuracy of results, and smooth modelling.

If a project is loaded and the project datum hasn’t been set, a nag will appear in the Output Window warning that the project datum should be set.

To show the datum of a project simply do the following:

  • Load up a piece of project topography.
  • Run the command File/Project datum/Show project datum.
  • The geometry will be unloaded, the project datum will be written to the output window. Now, when geometry is loaded the warning messages will no longer appear.

From the user perspective, all geometry will be saved and loaded in the correct coordinate system and no further thought needs to be given to this issue

The same surface, after the project datum was set, was contoured at the same interval, and the results are below. Contours no longer overlap.

Similarly, the point markers are displayed correctly:

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