By Charlie Frye, Esri Chief Cartographer
When an elevation raster dataset contains values for both topography and bathymetry, often the best solution to this is to split it into two new rasters: one for the topography and one for the bathymetry. The reason is that symbolizing this data is difficult. This is because a color ramp that contains appropriate hues for both bathymetry and topography must be adjusted so that it shows the shoreline exactly in the right place, at zero elevation. We have created a geoprocessing model that splits a raster elevation dataset into two datesets based on a split value you assign. Thus, this solution is ideal for small and medium sized elevation datasets as it offers the ability to precisely symbolize the land water interface. The idea is to split the dataset at zero elevation and the use a land based color ramp for the output topography dataset and a water based color ramp for the output bathymetry dataset.
We have also created a style file (Topo_Bathy.style) that contain color ramps for a variety of elevation models. These are a few of the color ramps already on Mapping Center’s ArcGIS Resources tab and are provided here to get you started.
We will be posting several more blog entries on symbolizing terrain in the coming weeks (including strategies for symbolizing the land water interface on large elevation datasets as well), and will highlight these as well as the other color ramps that are available as in those entries. This is also a good place to note just who the “we” in this case represents, Aileen and I have been working with Witold Fraczek, who is an Application Prototype Specialist here at ESRI with a great deal of experience working with and producing terrain representations. Witold also created the color ramps in Topo_Bathy.style file.
Here is how to use the model:
1. Download the model and the style; they are inside the same ZIP file.
2. Familiarize yourself with the model – open the model in ArcCatalog; inside the SplitRasterAtValue toolbox is a model named Split Raster at Value. Right-click on it and choose Edit. The model should look like figure 1.
Figure 1. The model
The model’s parameters are:
- Split value: the values above this number will be stored in the above output dataset, and the values below or equal to this value will be stored in the below output dataset. For a typical DEM, this would be set to 0 (zero).
- Input raster: Browse to your DEM.
- Output workspace: This is the workspace, a folder in this case, where you want the output datasets to be written.
Witold also gets the credit for the simplicity of this model as he figured out how to use the Con tool in this case.
Once the model has run, symbolize the output using the color ramps supplied in the Topo_Bathy.style file.
In addition to splitting elevation data at the shoreline, this model could be used to split elevation models at the treeline or snowline. Further, this model could be used to split statistical surfaces, allowing them to be easily symbolized with color ramps that match exactly to specific data ranges.