...
| Info | ||
|---|---|---|
| ||
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 similar to a "nearest neighbor" one . This technique assigns (i.e.. assigning 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 However, GRASS is not using a dedicated "nearest neighbor" function in that case, but rather applies a "pixel mask" of the resolution of the DEM over any other raster, and assign the values of the other raster to match the resolution of the DEM. We were not able to specify exactly how GRASS proceeds to do that, so we should not assume the process is a clean "nearest neighbor" attribution. It is therefore strongly encouraged to always ensure that the input raster files have the exact same resolution. While change of raster resolution may likely be correct for categorical raster such as the the landcover, this is not the case for the population distribution layer as this transformation will not conserve the original total population and can therefore 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,
| Caption ref | ||
|---|---|---|
|
...
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.
...
- A situation in which the density of artificial bridges to be corrected is low (as indicated in red in
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:Caption ref anchor bridges - Generate a buffer with a radius equivalent to 1.75 time the resolution of the original land cover grid (in light blue in
b figure b below);Caption ref anchor bridges - Put the hydrographic network layer in the editing mode and move the concerned segments outside of the buffer area as presented in
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 inCaption ref anchor bridges
c figure c below;Caption ref anchor bridges - 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 figure d below.Caption ref anchor bridges
Captioneditem anchor bridges caption 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)2. A situation in which the density of vertex on the segments to be corrected is dense (indicated in red in
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:Caption ref anchor hydr_correc - Generate a buffer with a radius equivalent to 1.75 time the resolution of the original land cover grid (in blue in
b figure b below)Caption ref anchor hydr_correc - Put the hydrographic network layer in the editing mode and add a new river segment outside of the buffer area as presented in
c figure c below (light blue line)Caption ref anchor hydr_correc - Save your edits and run the first module of AccessMod. The new resulting merged land cover should then look like presented in
d belowCaption ref anchor hydr_correc
- Generate a buffer with a radius equivalent to 1.75 time the resolution of the original land cover grid (in light blue in
...
- figure d below
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
| Caption ref | ||
|---|---|---|
|
...
| anchor | raster_roads |
|---|---|
| caption | Examples of rasterization of a vector format road segment (a) for a resolution of 180 meters (b) and 1 kilometer (c) |
...
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
| Caption ref | ||
|---|---|---|
|
...
The solution to address this problem is to:
...
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
| Caption ref | ||
|---|---|---|
|
...
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).
...