We’ve seen how we can look “through” several data layers to analyze the spatial and attribute coincidence through “overlay.” Now how about functional relationships that concern the distance or space on the map. We start with buffers.
Buffers – the distance from point and line features
We can associate every location on a map with its distance to and association with lines, features or polygons, or raster features. When we look for areas described by a given distance or set of distances from a feature, we are making a “buffer” of that feature. Every GIS tool has the possibility of constructing a single, and usually, multiple distances from a point, line or polygon.
Here are single and multiple buffers of some stream segments near Lexington. Imagine needing them for water quality control, or to study groundwater interactions with streams.
Here are single and multiple buffers of some stream segments on streams west of Lexington. Imagine needing them for water quality control, or to study groundwater interactions with streams.
 a single distance buffer (250 m) |
 a multiple distance buffer (each ring is 150 m) |
From the demo/bufferQ folder, open the project and use the vector “buffer” tool to construct a single buffer. To construct a multiring buffer, note the difference in the the native Q tool (doesn’t dissolve the buffers) and the “Multi Ring Buffer” plugin.
Distance Maps
If what you need is not a discrete distance from a feature, but an analysis of “how far”, then you want a surface that is a “distance map” showing the distance to a feature for every point on the map. This might come up if you were modeling pollution and need to know how far every point is from a road to determine atmospheric lead fallout. In QGIS, the easiest way to build a distance map is the first Rasterize (SAGA Function) your input layer if it is not already a raster feature. Then, you run the Proximity (Raster Distance) tool, taking care to choose the option under the dropdown “Coordinates” that reports distance in map units…we humans like to measure distance in meters or feet, not pixels. An example of this applied to our buffered stream from above can be seen here:
Once you have made the distance raster, you can select any distance, or group of distances, to use as a buffer (in either raster form or by converting back to vector).