DEM-Hydro Processing scripts

Aggregate statistics

This script allows you to cross a merged catchments map with a DEM (or any other map): you will obtain for each catchment the following values: average, minimum, maximum, std, predominant, median, sum, count.

The results are added as new columns to your existing merged catchments table.

Dialog box options:

Merged Catchment Table:	Select the output table of a previous Catchment Merge operation. The output columns that will contain the result of the script will be added to this table.
Merged Catchment Map:	Select a raster map that is the output of a previous Catchment Merge operation. The map may contain a single catchment or multiple catchments.
Parameter Map for Aggregate Statistics:
	Select a raster map with a value domain, for instance your DEM. The merged catchments map will be crossed with the raster map selected here.

Tip:

For more information, refer to Table calculation : Joining columns from other tables.

Note:

If you choose a class map for the cross (instead of a value map), the join operation cannot aggregate the classes very well. Then, the predominant class 'value' will be returned.

Output columns:

Suppose you cross the merged catchments map with a DEM:

average	The average height value in each catchment.
minimum	The minimum height value in each catchment.
maximum	The maximum height value in each catchment.
std	The standard deviation of height values in each catchment.
predominant	The predominant height value in each catchment.
median	The predominant height value in each catchment.
sum	The sum of all height values in each catchment.
count	The number of pixels in each catchment.

Short explanation of the calculations by the script:

First, the Merged Catchments map is crossed with the other raster map (value domain); the results are stored in a temporary cross table.
Subsequently, the Merged Catchments table is opened, and several Join operations are performed:

the values of the DEM are retrieved from the temporary cross table,
these values are aggregated by the IDs of the merged catchments (to obtain a single value per catchment),
during some Joins, the NPix column is used from the cross table as a weight factor,
the result of each Join operation is stored in a new column in the Merged Catchments table.

To prepare for the calculation of the summed values per catchment, first a calculation has to be performed in the temporary cross table:

temporary column temp_sum = values_2nd_map * NPix_column_in_temp_cross_table

The Joins are performed by the following expressions:

output column average = ColumnJoinAvg(temp_cross_table, values_2nd_map, merged_catchment_IDs, NPix)

output column minimum = ColumnJoinMin(temp_cross_table, values_2nd_map, merged_catchment_IDs)

output column maximum = ColumnJoinMax(temp_cross_table, values_2nd_map, merged_catchment_IDs)

output column std = ColumnJoinStd(temp_cross_table, values_2nd_map, merged_catchment_IDs, NPix)

output column predominant = ColumnJoinPrd(temp_cross_table, values_2nd_map, merged_catchment_IDs, NPix)

output column median = ColumnJoinMed(temp_cross_table, values_2nd_map, merged_catchment_IDs, NPix)

output column sum = ColumnJoinSum(temp_cross_table, temp_sum, merged_catchment_IDs)

output column count = ColumnJoinSum(temp_cross_table, NPix, merged_catchment_IDs)

Finally, the temporary cross table and other temporary maps are removed, and the output merged catchments table with the new columns is displayed.