La première étape nécessaire consiste à obtenir la couche d'occupation du sol au format raster utilisée dans AccessMod en fusionnant le réseau routier, le réseau hydrographique (rivières et plans d'eau) et la couche d'occupation du sol de base. Ceci est fait en utilisant l’outil “Fusionner l'occupation du sol” présent dans le module Outils d’AccessMod.
La fusion de ces différentes couches peut être réalisée en dehors d'AccessMod à l'aide d'un logiciel SIG prenant en charge des données au format raster, mais le processus peut être relativement compliqué, et donc fastidieux, en particulier si des priorités doivent être attribuées aux différents types de route et/ou si des artefacts générés par le fusionnement doivent être nettoyés (voir Section 3.3.1.1). C'est pourquoi AccessMod offre un moyen simple, mais complet, d'exécuter efficacement le processus de fusionnement, tout en tenant compte de ce qui précède.
Dans l'exercice suivant, nous allons utiliser AccessMod pour fusionner les différentes couches précédemment importées, afin d'obtenir l'occupation du sol raster finale utilisée dans les analyses suivantes.
Le fusionnement se fait par la mise en œuvre des étapes suivantes:
- Ajouter les différentes couches dans la pile
- Mettre dans le bon ordre les différentes couches incluses dans la pile
- Fusionner les couches ordonnées pour obtenir la couche d'occupation du sol finale
Ces étapes sont décrites plus en détail dans les sous-sections suivantes.
Ajouter les différentes couches dans la pile
Commencez par sélectionner l'outil "Fusionner l'occupation du sol" dans le module "Outils". Ajoutez la couche d'occupation du sol d'origine à la pile à l'aide de l'onglet "Occupation du sol" et suivez les étapes suivantes:
(1) Sélectionnez la couche d'occupation du sol d'origine disponible dans la liste déroulante.
(2) Choisissez la table de classification de la couverture du sol (voir la section 3.3.2.1) qui fait le lien entre chaque classe d'occupation du sol (nombres entiers) et l'étiquette correspondante (chaînes de caractères). Si une table est sélectionnée (comme illustré dans la figure ci-dessus), son contenu est chargé et affiché dans le tableau du bas de la section "Etiquetage des classes d'occupation du sol".
(3) Sélectionnez "Importer les étiquettes du tableau ci-dessous" afin de copier les valeurs de la table inférieure dans la table supérieure. Cette table supérieure est modifiable et vous permet donc de modifier le libellé de chaque catégorie si nécessaire. Cependant, les identifiants de classe ne peuvent pas être modifiés.
(4) Cliquez sur "Ajouter à la pile" pour ajouter cette couche à la "pile" de couches à fusionner.
Ajoutez ensuite la couche de route à la pile à l'aide de l'onglet "Routes" dédié, en procédant comme suit:
(1) Select the road class vector layer from the drop-down list; this should pre-populate the attribute table on the right-hand side of the screen
(2) Select the field from the attribute table of the road layer that contains the different road classes (INTEGER value).
(3) Select the column from the attribute table of the road layer that contains the labels for the different road classes.
(4) Click "Add to the stack" to add each road type as a separated layer in the stack.
AccessMod will automatically reclassify road classes that present a value below 1000. This is done in order to avoid any overlap with classes that could be used in the original land cover layer. This situation is illustrated in the above figure where the road classes being used are 1, 2, and 3.
This is the reason why the above image includes a message just above the "Add to the stack" button "Some classes values are less than 1000. AccessMod will automatically convert them."
The next step is to add barriers to movements (i.e. areas that cannot be crossed by the population). In the present exercise, two layers are added through the use of the "Barriers" tab: rivers and wetlands (water bodies):
To add rivers as barriers in the stack:
(1) Select the "barriers" class vector layer that contains rivers from the drop-down list.
(2) Verify that the barrier type corresponds to "Lines" in this case (note that the selection is automatic based on the majority of feature types (lines, polygons, points) in the vector data set, as displayed in the table (3)).
(3) The content table here does not allow for editing and indicates the number of objects contained in the selected barrier layer based on the type (points, lines and polygons).
(4) Click on "Add to the stack" to add this layer to the stack.
To then add the wetlands as barriers as well:
(1) to (4) again, using the "barriers" class vector layer that contains the wetlands.
Once these two steps are done, the different layers in the stack can be ordered and merged to obtain the merged land cover layer.
Ordering and merging the different layers the stack
The "Merge" tab allows users to order the layers before merging them.
The objective of the ordering is to define which layers from the stack will be overlaid on top of the others. This is important to define, for example, if the roads are allowed to cross rivers (bridge) or need to be considered as under water (i.e., flooded).
At the same time, the possibility to order the different road classes in the stack allows users to define which road class will have the priority over other road classes when there are cross-roads. This is of particular importance when the datasets used are at low resolution.
The ordering will be performed as follows in the context of the present exercise considering that roads allows crossing barriers (bridges):
(1) Place the road layers on top of the two barrier layers, and with the landcover layer at the bottom of the stack to simulate roads passing over the rivers and wetlands (bridges), and rivers and wetlands representing natural barriers to movement in the landcover. You can order each layer by clicking on it, holding the left button of the mouse and moving it to the desired place in the list.
(2) You can use the buttons on the left-hand side to:
- Define items to be skipped during the merge after dragging and dropping them in the "Stack items to skip" part of the section. These items will not be included in the merged land cover layer. This option could for example be used to temporarily remove one particular type of roads from the stack and therefore the resulting merged land cover layer
- Skip all the items from the stack by clicking on the "Skip all items" button on the left. However, doing this would not allow creating a merged land cover layer as the stack would be empty.
- Delete the items placed in the "Stack items to skip" part of the section by clicking on the "Delete skipped items" button on the left. However, this action cannot be reversed; the layer in question will have to be added once again to the stack if needed
- Bring all the items placed in the "Stack items to skip" list back into the stack by clicking the "Use all items" button on the left
(3) The "Clean artefacts (this can take some time)" option allows users to make topological corrections after adding roads to the stack. In Figure 9 we can see that adding the layer of roads produced an artificial bridge (artefact) between cell e5 and cell d6, which appears to cross the river, while the road actually does not cross it. AccessMod can automatically remove these kinds of artificial bridges.
If you choose this option, AccessMod will output an additional raster data set that will show the locations of the cells that were identified as artificial bridges and that were removed by transforming them to barrier cells. The name of this additional data set is the name of the merged landcover, plus "bridge" (in our case here, it will be called "landcover merged bridge"). In this data set, cells that were artificial bridges have a value of 1, while all other cells have "No Data" values.
You can see this data set in the Data table (after the land cover merge process) only if you have selected "Enable advanced options in modules" in the AccessMod "Settings module" (see Section 5.9.1 below), and if you have selected “Show internal data" in the "Filter" section of the "Data" module. Please note that applying this topological correction process can significantly increase calculation time especially on big datasets, because each cell has to be processed by the algorithm.
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(4) Give tags to the final merged land cover layer, "landcover merged" in the present example.
(5) Hit "Merge the items in the stack" to merge all layers that are listed in the "Stack items to use" list.
A transparent window with some text and a progress bar will appear in front of the panel while the merging is occurring. Please wait until this window disappears to continue using AccessMod. Once the merging process has finished, go back to the "Data" module where you will see the new layer "landcover merged" appearing in the list, ready to be used for the different analyses:
To verify that this merged land cover layer is the desired one, you can visualize it by going in the "Raster Preview" tool of the Toolbox module, and by selecting this layer in the drop-down menu:
Note that the "Raster preview" module is not intended to be a fully-fledged GIS visualization tool, and its functionalities are limited. If you want to check more thoroughly if this layer has got the intended raster values, it is highly recommended to export it from AccessMod using the data module, import it in your GIS software and check its content before using it for the different analysis offered by AccessMod.