Inverse Distance Weighting (IDW) Interpolation using QGIS
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In this tutorial we will use a sample set of point locations to create a raster surface based on the data's attribute values. To do this we will use the IDW interpolation method.
WEIGHTING CRITERIA MAPS
Weighting of the map layers is achieved using a portion of the Analytical Hierarchy Process (AHP) developed in the early 1980s as a systematic method for comparing decision criteria.
3 Weighting with inverse distances
This simple method uses for weights some power of the inverse distance:
where p=1,2 or 3.
Other weighting systems are used occasionally - for example, in share trading a volume weighting will weight each time period in proportion to its trading volume.
to use equal weightings:
0.33 * red + 0.33 * green + 0.33 * blue
The # operator has three other forms: r#map, g#map, b#map. These extract the red, green, or blue components in the named raster map, respectively. The GRASS shell script r.
4. DISTANCE WEIGHTING - ASSIGNS MORE WEIGHT TO NEARBY POINTS THAN TO DISTANT POINTS.
A. DEPENDING ON THE SPATIAL LOCATION OF RAW DATA AMBIGUITY CAN RESULT DUE TO CHOICE OF WEIGHTING FUNCTION.
The adequacy of the initial network weighting described above is indicated by the variance of unit weight, which equals the square of the standard error of unit weight. The variance of unit weight should range between 0.5 and 1.
Weighting provides a common measurement that is broad enough to accommodate the varied missions of the different agencies, while also including the specific considerations and resource impacts that can occur in a particular FPU.
2] are amplitude weighting factors, chosen in such a way to match (as closely as possible, when both data and pilot channels are considered) the PSD of Equation (1), with [[w.sup.2].sub.1] [[w.sup.2].sub.1] = 1.
Infiltration and runoff methods: SCS Runoff, CN with automatic CN weighting, Loss, Green and Ampt, Horton (User Defined), EPA SWMM Runoff
Time of concentration methods: User-Defined, Carter, Eagleson, Espey/Winslow, Federal ...
Basic methods include inverse distance weighting: this attenuates the variable with decreasing proximity from the observed location.
The simplest form of weighting is Inverse Distance Weighting, or IDW. This technique creates weights according to the distances between the interpolated location (x, y) and each of its neighbors.
The same data have been interpolated to a grid with the Inverse Distance Weighting function.
If you want to play with weighting these parameters differently, you can multiply or divide them by wights in the raster calculator:
([accessibility] / 1.3) + ([viewshed] * 1.5) + ([snacks] * 3) ...
It also offers the advantage of allowing the surface smoothness and fit to be adjusted by selective weighting of the equations corresponding to the known elevations, thereby allowing grid adjustments to be made on a node-by-node basis if desired.
This value can be calculated by performing statistical measures and weighting the data.
The criteria don't necessarily have to be specifically state in the RFP, but this information can be useful for bidders. Identify your selection criteria in order of importance, possibly even weighting various factors and developing a scoring scheme ...
 See also: Weight, Analysis, Model, Information, GIS
  
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