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It is important to bear in mind that input vector data sets such as the road network and barriers to movement (e.g., rivers, bodies of water) are converted to grids when generating the merged land cover distribution grid. This conversion is done using the same resolution as the original land cover distribution grid. Depending on this resolution, the conversion can have a direct impact on the spatial relationship between the roads and the barriers in the merged land cover layer.

As a first example, Figure 1 

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 below shows the result of the conversion (rasterization) of a road (black) and river (blue) layer for different resolutions. As we can see, the lower the resolution (i.e. larger raster grid size), the higher the risk to generate an overlap between roads and the rivers, therefore creating artificial “bridges” (red arrows) that do not exist in reality.

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Example of rasterization of a road (in black) and river (in blue) segments (a) at a resolution of 180 meters (b) and 1 kilometer (c) with the indication of artificial passages (red arrows) that gets created

AccessMod 5 contains a new function which aims to clean most of these artifacts (see 0). Even so, it is important to be aware of this issue and to check the result of such cleaning because the existence of these artificial “bridges” (passage in the merged land cover layer that does not exists in real life) can greatly influence the result of the accessibility analysis.

In case too many artificial bridges remain after the application of the above mentioned function, two approaches are possible. The approach chosen will depend on the original hydrographic network layer:

1. A situation in which the density of artificial bridges to be corrected is low (as indicated in red in Figure 2 below, road in green and river in white). In this case, the user should open the merged land cover, the road and the hydrographic network layers in a GIS software and:
   1. Generate a buffer with a radius equivalent to 1.75 time the resolution of the original land cover grid (in light blue in Figure 2b below);
   2. Put the hydrographic network layer in the editing mode and move the concerned segments outside of the buffer area as presented in Figure panel 2c below (highlighted blue line). You might also want to adjust the way the hydrographic network cuts the road network by placing them perpendicularly as also shown in Figure 2c below;
   3. Save your edits, import the new layer in AccessMod and run the merge land cover tool once again. The new resulting merged land cover should then look like presented in Figure 2d below.

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   Example of correction process operated on the hydrographic network with low density of artificial bridges with rivers showing up in white and roads showing up in green (see text above for explanations)