CatchmentSIM is a terrain analysis package for raster DEMs. Thus, forming a DEM is a fundamental part of any analysis. The CatchmentSIM DEM can be interpolated from 3D contour and 2D stream alignment data, imported from an external software package or a blank DEM can be created in CatchmentSIM and populated based on an external surface (raster or TIN), called sampling.
In the case of interpolated or sampled DEMs (but not imported), it is first necessary to setup a blank DEM. This is undertaken using the Setup Blank DEM form (Common >> Setup Blank DEM). The first requirements are to setup the DEM boundaries and resolution, as outlined in the following section.
DEM Boundaries and DEM Resolution
The rectangle defining the external boundary of the DEM can be set by the user to be any size equal to or smaller than the selected project boundaries, and may have any number or rows and columns. The software will generate a warning if it determines that the DEM characteristics are outside of the user's computational resources. CatchmentSIM does not require DEMs to have square pixels and can accommodate rectangular grid pixels. However, an option is available to ensure that the DEM has square pixels. If this option is selected then the number of DEM columns will be automatically calculated after the user enters the number of DEM rows to ensure a square pixel DEM is developed. The user can also elect to form a DEM based on an explicit pixel size and CatchmentSIM will determine the required number of rows and columns.
| Note: | Although CatchmentSIM supports rectangular pixel DEMs, many other software packages do not. You should use a square pixel DEM if you wish to export the DEM to other software packages. |
In order to maximise the precision of the analysis within the available computational resources, the DEM boundaries should be chosen carefully to ensure the smallest possible DEM that contains the entire catchment is created. The total number of pixels should be maintained in a reasonable range. For example, a project with 1 million pixels (eg., 1000 rows * 1000 columns) requires about 10 -12 MB of hard disk space (for all project files) and the most computationally intensive algorithm requires about 3 minutes to process (on a Pentium 2.4 GHz). The computational demands of the software in terms of storage space and processing time will both increase with the number of pixels in the DEM. However, the accuracy of any subsequent hydrologic analysis can be related to the grid pixel resolution (Goonetilleke and Jenkins 1996). As such, the DEM boundaries and number of rows / columns should be chosen carefully as a balance between desired accuracy and computational resources.
However, if watercourse data is planned to be incorporated into the model (in a contour / stream alignment interpolation - recommended if available) it is important to set the project boundaries so that the next contour line intersection point along the main stream downstream of the desired catchment outlet is included within the project boundaries. This ensures that the drainage enforcement algorithm is able to be applied to the entire reach of the main stream within the catchment.
The DEM precision can also be set in CatchmentSIM (CatchmentSIM Drop Down >> Project Options | Digital Elevation Model). This refers to the number of significant figures to which elevation values are recorded. The user may choose between single precision (7-8 significant digits) and double precision (15-16 significant digits). The use of double precision will double the hard-disk and memory requirements for the DEM file. Double precision storage of DEM values may only be necessary in regions with large vertical ranges or values that are recorded in small increments such as feet. For example, if single precision is being used and elevation values reach 10,000 then only 2-3 decimal places may be recorded which may be insufficient to represent very gradual slopes if the horizontal resolution is high.
The following sections apply to DEMs that will be interpolated from contour and stream alignment data. The next relevant section for DEMs sampled or imported is Hydrologic Conditioning.
DEM Interpolation
The minimum data requirement for interpolation of a DEM is 3D contour data. The accuracy and fitness for the purpose of such a DEM will be primarily a function of the level of contour definition of the imported data and required spatial resolution of the model. However, the DEM quality may be greatly improved by using a vector watercourse layer in conjunction with the interpolation algorithm. These data layers are utilised by the DEM interpolation algorithm in a number of sequential steps, namely:
1 Vector to raster conversion of 3D contour lines;
2 Incorporation of watercourse GIS layers (optional);
3 Implementation of interpolation aids (optional);
4 Interpolation of raster DEM; and,
5 Stream burning (optional).
These algorithms are described in the following sections and the relevant menu items are described in the Create DEM section of the Program Reference section.
