Step by Step: Recommended Practice Exposure Mapping

Data Access: 

Population data from WorldPop is available free of charge and covers Africa, Asia, and Central and South America. The data can be downloaded without registration from the WorldPop website. Landcover data can be obtained from Globeland 30 free of charge. A registration for downloading data is required.

 

The flood mask was created by the Recommended Practice Flood Mapping and its step-by-step procedure (Link)

 

Processing and screenshots are based on Quantum GIS version 2.10.1-Pisa 64-bit. In the case the Zonal Statistics function is missing, use the "Manage and Install Plugin" tool to install the missing function.

 

Population

Step 1: Data preparation

Step 2: Geospatial analysis

Step 3: Visualization

 

Landcover

Step 1: Data preparation

Step 2: Geospatial analysis

Step 3: Visualization

 

Data Preparation/Pre-processing: 

Population

1. Data preparation

1.1 Download population data for Malawi for the year 2015 from the WorldPop website (MWI_pph_v2b_2015_UNadj). The data set has a 100 x 100 meter resolution (1 hectare). As each pixel contains the population per hectare value, each pixel also represents the respective total population. SNAP image

1.2 Load the water band (be sure it has the same projection as the population data (UTM)) and the population data in Quantum GIS. SNAP image

1.3 Polygonize the water band to receive a shape file containing the water polygons. Open Raster --> Conversion --> Polygonize and define a new .shp file. SNAP image

1.4 To delete the background of the water band (no water), first open the Attribute table with a right click on the water_VV_vector file on the left side and the selection of Open Attribute Table. SNAP image

1.5 In the Attribute table, use the Select by expression function to select all the non-water entries (DN = 0). To delete all the selected features, use toggle editing mode and click on the litter box. SNAP image

Processing Steps: 

2. Geospatial analysis

2.1 The Zonal statistics function (Raster --> Zonal statistics) can now be used to receive the population data solely for the water polygons. Select the population image as raster layer and the water polygons for Polygon layer containing the zones. Define an output column prefix and check Sum. SNAP image

7. The Basic statistic function (Vector --> Analysis Tools --> Basic statistics) enables the summarization of all sum entries in the water polygon, which is the total population in the area of interest. The comparison of the water band with the population results in 9,136 affected persons. Note that this number is affected by uncertainties. SNAP image

3. Visualization

3.1 To create a quickly interpretable exposure map, change the filling of the water mask to transparent (Right click on water_VV_vector --> Properties --> Style). Adapt the colors of the population data set (Right click on MWI_pph_v2b_2015_UNadj --> Properties --> Style): Change Render type to Singleband pseudocolor and choose a color bar.

3.2 To create a map with legend, scale bar, and north arrow, you can use the Composer function (Project --> New Print Composer). Example of a final exposure map in the context of population: Image

 

Landcover

1. Data preparation

1.0 Load a shapefile with extent of a natural disaster (water_VV_vector - in UTM format), landcover information, and country boundaries in Quantum GIS. Landcover information can be obtained from GlobeLand 30, in our case it is tile S36_15. Country shapefiles can be downloaded at http://www.gadm.org/country. Note: The boundaries and names of the shapefiles do not imply official endorsement or acceptance by the United Nations. Image

1.1 Change the projection of the boundaries from geographical to UTM. Right click on MWI_adm0 and use the Save as function to save the new file with UTM coordinates (same zone as the disaster map) SNAP image

1.2 Clip water_VV_vector with the countries boundaries (MWI_adm0_UTM). Open Vector --> Geoprocessing Tools --> Clip and define a new .shp file. Note that this step can take considerable time (several hours) depending on the size of the disaster map and the hardware. Image

1.3 Use the Raster clip function to clip the landcover raster data to the extent of the flood mask. Open Raster --> Extraction --> Clipper and define a new .tif file. Set the No data value to 0 and the clipping mode to Mask layer with the clipped water vector. Image

2. Geospatial analysis

2.1 Polygonize the clipped landcover data. Open Raster --> Extraction --> Clipper and define a new .shp file. Set the Field name to Landcover. Image

2.2 Now the geometry (size of each polygon) can be added to the shapefile: Vector --> Geometry Tools --> Export/Add Geometry Columns. Each polygon has now a label for Landcover, Area (in meters) and Perimeter (in meters). Image

2.3 Finally the area of the affected landcover can be estimated. Settlements are within the artificial surface class (Globeland code 80) and agricultural areas within the cultivated land (Globeland code 10). Note that the classes are generalized, i.e., Artificial surface are defined as: "Lands modified by human activities, including all kinds of habitation, industrial and mining area, transportation facilities, and interior urban green zones and water bodies, etc."; Cultivated land are defined as: "Lands used for agriculture, horticulture and gardens, including paddy fields, irrigated and dry farmland, vegetation and fruit gardens, etc.".

To estimate the area of each class, open the attribute table: Right click on GLC30_clipped_water_vector. Then select all polygons with the Landcover code 10 by using the Select by expression function (Landcover = 10). Image

Then by using the Basic statistics function (Vector --> Analysis Tools), the sum of all selected polygons can be calculated. Be sure to tick "Use only selected features". After receiving the area for class 10 (cultivated land), step 2.3 has to be done again for Landcover class 80 (artificial surface). Note that the results are in square meters, to get hectar simply divide the number by 10000. So in our case, 1296 ha of cultivated land and 48 ha of artificual surface were flooded. Image

3. Visualization

3.1 Analog to the visualization steps of the population section above, you can create a land cover map with the flood as an overlay. Change the filling of the water mask to transparent (Right click on water_VV_vector --> Properties --> Style). Adapt the colors of the population data set (Right click on s36_15_2010lc030 --> Properties --> Style): Change Render type to Singleband pseudocolor and adjust the settings depending on the user needs.

3.2 To create a map with legend, scale bar, and north arrow, you can use the Composer function (Project --> New Print Composer). Example of a final exposure map in the context of landcover: Image

 

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