Distance Map
A distance map is a proximity analysis type (distance measure among objects, usually as
a length measure) presenting zones with specified width (distances) around one or more elements in
the map, as presented in the figure below.

As an example, suppose one wants to bound all urban lots which are 50 meters far
from a specified distance from a certain street. The purpose would be to know the city
impact, when a dangerous material is transported through the city.
The next figure shows two possible paths to create a distance map, using as a reference
elements like points, lines or polygons. Notice that it is possible to get a thematic raster
map (which can be converted into a vector format later) by slicing a numeric IL with the
distance grid representation, in this case the pixels will be grouped in slices associated
to thematic classes. As another option, implemented since the version 3.6, it is possible to
get a thematic map, however vector, defining the slices and the association with thematic
classes previously defined in the database.

The difference between generating a distances numeric grid or a vector thematic map
is in the output category selection, this is, numeric or thematic respectively.
The requirements for generating a distance map are the following:
- to identify over which representation the distance map will be executed (raster or vector)
- to identify the element (point, line or polygon), class or object that will be used
to generate the output IL. The operation is valid for thematic, cadastral, networks or
numeric ILs;
- to have a numeric category previously defined in the database, where the distances
grid will be stored. And also have a thematic category, with enough number of classes
to execute the grid slicing later;
- to have a thematic category, with enough number of classes to define the slicing,
generating a vector representation IL directly.
Creating a distance map:
- activate in the "Control Panel"
the input information layer, thematic, cadastral, numeric or network, from which the
distance map will be created;
- select the IL representation you wish and draw in one of the windows;
- select the IL in the main menu Thematic / Cadastral /
DTM or Network - Buffering... depending on the model that the active
IL belongs to;
- in Selection, the buttons Class, Element or Objects will be enabled if:
- the active IL is thematic:
- Class: all elements of the selected class will be considered
- Element: only the selected entity will be considered (Point, Line or Polygon)
- select an option;
- the active IL is cadastral:
- Object: for all objects with the same label
- Element:
only the selected entity (Point, Line or Polygon)
- select an option;
- the active IL is network
- Object: for all objects with the same label
- Element:
only the selected entity (Point, Line)
- select an
option;
- the active IL is numeric
- Element: only the selected entity (Point or Line)
- select an option;
- click in the screen over the desired entity. Each selection is presented in the
Selected Elements list;
- if you want to remove an entity, select an item in the Selected Elements
list and click on Remove;
- click on Category... to select a numeric category which will be used
to store the IL with the distance grid or a thematic category that will be used to create
the distance vector map. At this point the " Categories List" window is
presented, select one in the list and click on
Apply;
- if the selected output category is numeric:
- give the output IL name;
- in Grid Resolution, give the grid resolution value in X and Y
(meters);
- if the selected output category is thematic:
- give the output IL name;
- in Distance Slices, click first on Definition... to inform the intervals for the
distances slices. The "Slices Definition"
window is presented. Click on Execute
in this window after defining the slices;
- in Distance Slices, click on Association...
to make an association of the slices with thematic category classes selected. The
"Slice - Classes" window is
presented. Click on Apply in this window for
the association;
- type in Number of Points in the Curve: the desired value. Valid values
between 1 and 180. This value corresponds to the number of points to be created in an
180 degrees curve. Curve with smaller angles will have a number of points proportional
to this angle. If the distance slice increases it is recommended to increase the number
of points in the curve;
- click on Apply;
NOTE: Use resolution values smaller than the distance that will be defined later for
the slicing. Once the grid is generated, with the distance values, go back to the
"Control Panel", activate the IL that has the created grid above and click on the
DTM - Slicing... in the main menu, to create a distances
map.
See also:
Numerical Grids Slicing
Programming in LEGAL
Consulting a Database.
Other options in the SPRING geographical analysis
|