Mapping Table:Index Maps
An index map is a GRASS ASCII file that contains integer values in each grid cell. The data should follow the same shape or pattern as the WATERSHED_MASK file because each cell of the index map will be used to supply data for the corresponding watershed cell. The data cells outside of the watershed should contain the value 0. All data cells inside the watershed should be of integer value greater than or equal to 1. These values are ID numbers, and will correspond to IDs from a table in the Mapping Table file. It is from the index maps that the final structure and mapping of the data in the tables takes place. Developing good index maps is a key part in building a model that is easy to work with and modify.
An example of a watershed mask and an index map file:
This index map has three IDs, 1, 2 and 3. Any table in the Mapping Table file that uses this index map should at least have the IDs 1, 2 and 3. The table may have other IDs, but the associated values will not be in the final floating-point map generated internally by GSSHA. A table may refer to more IDs than an index map references, but an index map cannot contain the IDs that are not listed in the associated table. Allowing IDs in the tables not associated with the index maps is useful in running different scenarios; for example, pre and post project conditions. To change the model scenario, only the name of a different index map need be assigned to the table in the Mapping Table file.
Each index map used in the Mapping Table file is identified by its index map name, which is different than the index map filename. The index map filename is the name of the file on disk, referenced by the operating system. The index map name is an internal descriptive name used in the Mapping Table file to identify which index map is assigned to which tables. The index map filenames and the associated index map names are associated with each other at the beginning of the file in the index map declaration lines. These index map declaration lines follow the first line of the file, which identifies the file as a Mapping Table file.
- 12 Mapping Table