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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,

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anchor artificial bridges

 below the figure 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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AccessMod 5 contains a new function which aims to clean most of these artifacts (see Section 5.5.2). 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.

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1. A situation in which the density of artificial bridges to be corrected is low (as indicated in red in

   Caption ref
   anchor bridges

    belowthe figure 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

      Caption ref
      anchor bridges

      b figure b below);
   2. Put the hydrographic network layer in the editing mode and move the concerned segments outside of the buffer area as presented in

      Caption ref
      anchor bridges

      c figure c 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

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      anchor bridges

      c figure c 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

      Caption ref
      anchor bridges

      d figure d below. 

   

   CaptioneditemanchorbridgescaptionExample 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)

   2. A situation in which the density of vertex on the segments to be corrected is dense (indicated in red in

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   anchor hydr_correc

   a figure a below, road in green and river in white). In this case, 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 blue in

      Caption ref
      anchor hydr_correc

      b figure b below)
   2. Put the hydrographic network layer in the editing mode and add a new river segment outside of the buffer area as presented in

      Caption ref
      anchor hydr_correc

      c figure c below (light blue line)
   3. Save your edits and run the first module of AccessMod. The new resulting merged land cover should then look like presented in

      Caption ref
      anchor hydr_correc

      d figure d below

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anchor	hydr_correc
caption	Example of correction process operated on the hydrographic network with high density of vertex (see text above for explanations)

It is important to be aware that the choice of a coarse resolution can generate the merging of separate road segments that are close to each other. This is shown in

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anchor raster_roads

below the figure below where there are artificial intersections between road segments at low resolution (larger grid size). Unfortunately, no correction can be made in this case.

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Users also need to be conscious of the impact that the change in resolution has on the travel time in cell where patients are reaching the road network.

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In addition to the above, the rasterization of elongated water body polygons (and other barriers to movement stored as polygons) does not always generate continuous surfaces, unlike when rasterizing vector lines. Figure 5 below provides an illustration of such a problem. As we can see in

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anchor raster_elong

cfigure c below, working with lower resolution (larger grid sizes) will create discontinuities, therefore generating artificial bridges in these areas.

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The solution to address this problem is to:

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It is important to have a clear understanding of the level of accuracy of the different geospatial data used for the analysis. As can be seen in

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anchor catchm_rivers

belowthe figure below, a facility located on the wrong side of a river will lead to the design of a completely different catchment area and therefore contribute to a different population coverage.

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In view of the above, working at the highest resolution possible would seem to be the best option. Nonetheless, it is important to remember that increasing the resolution of the raster layers results in an increase of the size of the files and the required computational time (see section 3.2.5).

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