Surface Area Take Off
The Area Take Off (ATO) option allows automatic calculation of runoff surface areas and the contributing areas of subcatchments using data imported from a GIS.
Full details of the theory behind surface area take off can be found in the Area Take Off Technical Note.
The following are required when carrying out an ATO:
- Checked out model that has subcatchment areas included
- An Impermeable Area Survey (IAS) drawing
The Impermeable Area Survey (IAS) to be used with the ATO function, is a coded, digitised map of the catchment's impermeable area.
The IAS is incorporated into the model by selecting Subcatchment | Area take off from the Model menu. This displays the Area Take Off Dialog.
The dialog box allows the digitised GIS IAS data to be read by:
- Browsing for a GIS file
- Selecting a Layer in the GeoPlan View
- Selecting polygons imported from GIS on Category Field
The IAS Runoff Surface number should be mapped to the Polygon User Number field. The IAS System Type field should be mapped to the Polygon User Text field.
Using the Area Take-Off Function
To carry out Area Take-Off:
- Click on
or choose Grid
windows | New subcatchments window from the Window
menu to open the Subcatchment Grid Window of the
Subcatchments Grid. - Select the entire Contributing Areas column and right click on one of the selected cells. Use the Flags option on the popup menu to set the flag to #G (geo). This will change the cells to the #G colour and delete existing values. Runoff surfaces do not need to be flagged. These will be adjusted automatically. To update selected subcatchments only, follow this procedure:
- Select all the subcatchments to be updated
- On the Subcatchment Grid View of the Subcatchments Grid, go to the Grid menu and choose Group network selections at top of grid.
- Now go to the Contributing Areas column and select the contributing areas cell for all the selected subcatchments.
- Change the flag for the selected cells to #G (geo)
- Choose Subcatchment | Area take off from the Model menu. This displays the Area Take Off Dialog.
- Select the source of the Impermeable Area Survey data to be used. This may be a GIS file, a layer displayed on the GeoPla nWindow, or polygons that have already been imported from the GIS file.
- Choose other options as needed. If you followed the alternative procedure in step 2. above, check the Update Selected Subcatchments Only box. Checking for overlapping surfaces is recommended. (See Area Take Off Dialog for descriptions of the options).
- Click OK to complete the process. The contributing area is filled in for all (selected) subcatchments. Flags for changed values are set to #G
- If errors occur in the Area Take Off process, the log file is always displayed. Check the file and see what the errors are. Overlap errors, or objects in the data that are not polygons, are not necessarily a problem. Computation failures probably are. Check the reasonableness of the imported data on the Subcatchment Grid Window of the Subcatchments Grid.
