Tutorial related to hydraulic analysis with th DC Water Design Extension.
This tutorial shows how to calculate node demands using Thiessen polygons.
Thiessen polygons are also called Voronoi diagrams.
![[Note]](images/note.png) | Note |
|---|---|
The following requires the "Create Thiessen Polygons" extension which is available from http://arcscripts.esri.com. |
You will need a sufficiently developed network model ot apply the
steps below. Especially the contents of the field dc_id in the Junction theme
have to be unique. The example below will also require a supply zone
marked in a polygon theme. If your data does not fulfil these
requirements (yet),
you can use the DC Water Design Extension sample data. See
the section called “How to Obtain the DC Water Design Extension” for details where to obtain
the sample data.
Assumptions and design parameters:
| The map unit is meter. |
| The population density is 200 p/ha. |
| The average demand is 100 lcd. |
| The flow (demand) unit is l/s. |
![[Tip]](images/tip.png) | Tip |
|---|---|
It is also possible to use a buffer around the points instead of a polygon theme of the supply zone. However this is not covered in the example below. |
Example 3.1. Demand Calculation
Load the "Create Thiessen Polygons" extension.
Add the Junction and supply zone themes to
the current view (Unless they have been added to it already).
Activate the Junction theme.
Click on the button.
Select point field for polygon ID link: In the Build Thiessen
Polygons dialog, select dc_id in
the list. Click .
Select polygon theme for boundary: In the Build Thiessen Polygons dialog, select your supply zone theme. Click .
In the Output Theme File dialog, select location and filename of the Thiessen polygon shapefile. Click .
The new Thiessen polygon theme will be added to the current view.
Move the Thiessen polygon theme below the junction theme, but above
other polygon themes such as the supply zone. Note that the Thiessen
polygon theme covers the same area as the supply zone theme. There is
one Thiessen polygon for every Junction.
Activate the Thiessen polygon theme.
Open the Thiessen polygon theme table: Select → in the menu.
Select the dc_id field.
Select → from the menu.
In the Summary Table Definition dialog, select Area in the field list.
In the "Summarize by" list, select Sum.
Click on the button.
Click on the button.
A new summary table is created (sumX.dbf where
X is a number).
Select the dc_id field in the summary table.
Go back to the view, e.g. by selecting → from the menu.
Activate the Junction theme.
Open the Junction theme table:
Select → from the menu.
Select the dc_id field.
Join the summary table: Select → from the menu.
Start editing the table: Select → from the menu.
Select the demand field.
Open the field calculator: Select → from the menu.
In the [demand] field enter the formula to calculate the demand:
[Sum_Area]*100*200/10000/24/60/60
Click .
The nodal demand is now calculated.
Remove all joins: Select → from the menu.
Stop editing: Select → .
Save the edits: Click .
| Tip |
|---|---|
If you have a very large number of Thiessen polygons to create, the shpvoronoi commandline tool will provide better performance than the "Create Thiessen Polygons" extension. |