Units and projection
AccessMod uses formulae that works with variables expressed in the metric system. To avoid any potential consistency issue, it is necessary for all the geospatial data (raster and vector) to be projected in the same metric coordinate system before importing them in AccessMod.
Note
AccessMod will not import a DEM that is not in a metric coordinate system. An error message will appear in this case.
An error message will also appear in case you are trying to import a raster format layer that does not present the same coordinate system as the one of the DEM.
For vector format layers, the GRASS engine used in AccessMod automatically reprojects the data sets to fit the DEM coordinate system if this is not already the case.
Users are strongly encouraged to ensure consistency of the projection parameters across all the input layers before uploading them in AccessMod.
In addition, it is recommended that the projection used be an equal-area projection to avoid strong biases in the calculation of surface of the catchment areas and biases in distance along least-cost paths. Examples of equal-area projections can include Albers, cylindrical equal-area projection, Gall-Peters, Lambert cylindrical equal-area, Mollweide, and Werner projections.
Extent and resolution
All the raster format data used in AccessMod should present the same extent in terms of min/max Easting and Northing. If this is not the case, AccessMod will use the extent of the input DEM, and will not consider any data outside of this extent.
In addition, all input raster format layers must present the same resolution as the one of the DEM.
Note
The GRASS engine used in AccessMod will resample raster format layers presenting a different resolution than the DEM to make it match the DEM’s resolution.
The resampling technique used by the GRASS engine is the "nearest neighbor" one. This technique assigns the value in each "new" pixel based on the value stored in the nearest "previous" pixel.
While using such technique might be appropriate when applied to the landcover raster format layer this is not the case for the population distribution layer as this transformation will not conserve the original total population and can translate in wrong output statistics.
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,
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.
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.
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:
- A situation in which the density of artificial bridges to be corrected is low (as indicated in red in 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:
- Generate a buffer with a radius equivalent to 1.75 time the resolution of the original land cover grid (in light blue in b below);
- Put the hydrographic network layer in the editing mode and move the concerned segments outside of the buffer area as presented in 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 c below;
- 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 d below.
2. A situation in which the density of vertex on the segments to be corrected is dense (indicated in red in
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:- Generate a buffer with a radius equivalent to 1.75 time the resolution of the original land cover grid (in blue in b below)
- Put the hydrographic network layer in the editing mode and add a new river segment outside of the buffer area as presented in c below (light blue line)
- Save your edits and run the first module of AccessMod. The new resulting merged land cover should then look like presented in d below
