How do you calculate where to put the cable in your project?
When working on a renewable energy project, there is a need to connect to a suitable substation. When building a housing development project, there is a need to connect to a suitable grid. How is the route from project to grid calculated? More to the point, how can it be calculated automatically using a GIS?
With most project related analysis, there is a macro and micro scale analysis which needs to occur. In the macro (high) level, analysis will commonly be undertaken in GIS with input from a cable engineer and developer, who can provide detailed constraints and grid requirements to ensure the geospatial analysis uses the best information. Once a set of results have been derived for the macro scale, it will be plotted and analyzed in detail by all stakeholders to both alter the route, for it to be the most suitable, as well as to mitigate any detailed issues, such as habitats, species, utility operator nuances or local council stipulations.
Macro scale analysis
For this piece, focus will be on the macro scale geospatial analysis and how this is often undertaken. Having used this approach for several (successful) renewable projects and an offshore interconnector, it is tried and tested to work extremely well and can also be applied for other uses apart from cable routing.
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The approach uses a spatial analysis technique called “least cost path”, which works by allocating a weight to everything which may influence the potential course. The typical features could be (not exclusive to) topographic features like roads, buildings, trees, rivers and railway lines, as well as other features of influence like environmentally protected areas, airport exclusion zones, dense urban areas and historic or heritage areas.
Each of these considerations will be given a weight depending upon how complex it is to resolve them. For example, if it is impossible to pass through rivers or protected areas, then those conditions will be given the highest possible weight, however, other features like trees or forests will be given a high weight.
Creating raster
The overarching goal is to create a raster for the area of interest (AOI). Each cell will have a number associated to it, which will be derived from the features as layered raster. To begin with, use your favorite GIS and set the extent of the frame with your start point and end point. Considering my experience, it is always good to have some room around the start and end point to allow for ‘boomerang’ paths, because in some situations, a path is better than following a helical path into the end point.
Next, the most weighted features (assuming vector format) should be brought into the map, keeping the same extent. If there are more than one feature set of the same weight, then merge them together and rasterize the data. Ensure that a suitable cell size is used. If you use 25cm cells at a scale of 1:50,000 , it might take some time to process, unless it is running on a very small area. That is why nothing smaller than a metre can be used. Once rasterized, allocate a high number (999999 for example) to all the areas occupied by the feature (this can be done either at the rasterize conversion or using re-class raster). Don’t forget to allocate zero for all the other areas.
Now is the time to think about how to weight the features so that they are rasterized, and correct weights are allocated. Next, load the next most weighted feature(s) and rasterize them to the same extent as the first. Using the same procedure and cell size, allocate the cells that contain the feature with the chosen number, while keeping all the other cells as zero. Keep creating the raster until you have all the different levels of weight.
The next step is to merge all the raster together. It is best to use raster math (which is available in QGIS and ArcGIS Pro/Map). Using your chosen GIS, add all the raster together, each of the overlaying cells are added together to provide one final raster, which has the sums in each cell.
Note that there are several cost path tools in the Esri software(s), the one which I recommend is the “Optimal Path as Line”. This is found in the Spatial Analyst toolset and produces a vector line. It can have either a vector input for the start and end points or you can convert the start and end point to raster and allocate a unique value to the cell (not
used in the final cost raster). This is similar to the Grass r.cost tool, where a raster or vector feature may be used for the start and end points.
Bonus tips
Pro tip 1: If you need to adjust the cost weighted raster at any time, avoid doing that directly to the ‘final’ weighted raster which you created. Rather make copies of the raster in a new folder and adjust the necessary raster in the feature weighting, and then add them together. This might feel like a lot of work, but if you end up with a project manager, who wants to run different scenarios, it will help you to easily recall individual changes and go back easily.
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Pro tip 2: Rather than using a DEM (digital elevation model) for the terrain, convert the DEM to a slope raster and allocate cell values based on the complexity. For example, if you are looking at a travel path, a slope of more than 70 degrees would be almost impossible for the average vehicle. For technical outputs, it is always polite to add KPs (kilometre points) along the path, where it may run over several kilometres. This may be performed using the “points along line” tool, where the distance is set at 1km. This may then be merged with a point data of the points at the vertices of the line (with attributes for distance and bearing). For any technical cable or route analyst, this is useful for allocating key text.
