Difference between revisions of "General Considerations:Map Headers"

Latest revision as of 18:22, 3 July 2008

Many spatially varied inputs for GSSHA can be input as ASCII GRASS GIS raster maps. Raster maps are defined as maps that assign attributes to areas, as opposed to vector maps, which assign attributes to lines or polygons. GSSHA requires that all raster grid cells are square.

Each raster map, including index maps, must have a header that conforms to the following example:

north:	4156000.0
south:	4135000.0
east:	601800.0
west:	575000.0
rows:	105
cols:	134

The entries for north, south, east, and west are the coordinates of the bounding rectangle that contains the entire watershed. The rows and cols entry in the header respectively contain the number of rows and columns in the watershed. Note that for this particular example header, that (north-south)/rows = 200 m, and (east-west)/cols=200 m. Therefore, the grid size of this particular model is 200 m, and the raster cells are square, as required by GSSHA. As the coordinate values in the map header are not used to determine geographic position on the globe, any values for north, south, east, and west may be used in the map headers provided that the grid size is accurate, the grids are square, and your maps are consistent. If the grid size calculated from the bounding rectangle, rows, and columns is not equal to the value specified in the project file using the GRID_SIZE project file card, GSSHA will not run. The header in each GRASS ASCII map must be IDENTICAL. This requirement forces the user to ensure that all maps are of the same geographic region and grid size.

The header is followed by rows and columns of space delimited data values. These values can be either integer or real, depending on the data type. For example, the mask, channel link, channel node, and index maps are by definition integer maps; maps containing the land surface and bedrock elevations contain real values.