Trade areas

Parameters

Description :

The trade area module creates circular zones around poles with a given ray, counts land data on each area and compute the overlapping part of areas.

Lets take a simple example : the land is Communes of Indre & Loire, with population data. Poles are agencies with commercial surface for each (see example). Attention the order of input data is important : the first must be the poles data.

Following the output type choosen, this is the resulting processing :

Output = Continuous data The module will compute the total population associated with each agency. It will be the sum of all communes for this pole. If communes are in several areas, their population will be dispatched to the different agencies. Details of the computation is described below.	Population associated to the 5 agencies even if areas overlap:
Output = Areas map The module will build the different areas, one for each pole when there is no overlapping. If two areas (1 and 2) overlap, the full overlapping land will be considered as a new area with identifier "12". (in green in the image)	Each agency has its own area, and some new area are common to several agencies :
Output = Qualitative data on areas Areas are associated to a quality (the area identifier) and can be represented with colors or qualitative symbols.	One quality for each area :
Output = Continuous data on areas The data associated with each area is the total population. It can be represented with proportionnal symbols.	Total of each area :

Dispatching formula:

One commune that is covered by 2 poles areas (overlapping) will have its population dispatched in this manner:

• C is the commune
• P1 and P2 are the values associated to the 2 poles
• D1 and D2 are the distances between C and the 2 poles
• A1 and A2 are the attraction values of the 2 poles. They are computed this way (HUFF method) :
  A1 = P1/D1^2 (exponent is 2)
  A2 = P2/D2^2
• T is the total of all attraction values : T=A1+A2

Parameters:

The following values can be changed :

• Ray : you can choose the common ray for all areas (in map units)
• Exponent : the squared distance is the default, but you can change another value
• Compute from centroids : if this box is checked, the distance between the pole and the land polygons will be taken from the center of the polygon. If not the border will be used. It is more precise, but slower.
• Dispatching data on the surface
  If some communes are composed of several islands, this option has an effect on the computation.
  If the option is not checked, only the biggest polygone of the commune is used, and islands are ignored.
  If it is checked, each island will receveive a part of the population proportionnaly to its surface, and each island will be used independantly for the computation. At the end all island data will be gathered to make a global commune value.

Finally it is possible to extract all commune data : it is the dispatching of population for each agency. You can click on the button Export data table. Cartes & Données will build a tabular text file with each line containing a commune, and each column containing an agency. The value is the population of this commune associated with this agency.

Script :

2      module untyped_list ""
3        mod_type integer "103"
3        mod_subtype integer "533"
3        mod_name string "Zones de chalandise"
3        mod_dads integer_list ""
4          ? integer "9"
4          ? integer "4"
3        radius double "20"
3        exponent double "2"
3        take_centroid boolean "T"
3        distribute_data boolean "F"
3        output integer "2"