The geographic coverage analysis allows you to take into account the availability of services (i.e. the capacity of health facilities to attend to patients) and to consider this in addition to the physical accessibility constraints, in order to define the catchment area associated with each health facility.
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The following two screenshots illustrate how to enter data and set parameters for the analysis:
Data input:
Under:
(1) “Select population layer (raster)”, select the raster format layer containing the spatial distribution of the target population that you want to use from the scroll down list (the one named "population" in the present exercise).
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(10) "Select zone name (text), select the field from the attribute table of the zone boundaries layer that contains the name of each zone
Travel scenario:
(11) With respect to the accessibility analysis (see Section 5.5.3), this section is used to either import the content of an external scenario table and/or to manually modify the information reported in the columns for label, speed and/or mode.
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(8) Add short tags: Indicate short tags to be attached to the different outputs of the analysis. We will use "geographic analysis 120m" for the present analysis. Avoid very long name as it has been found to sometime prevent Excel to open the output generated xls files.
Facility selection
(9) As in the previous analysis, you can select the set of facilities for which the analysis will be performed. Keep all facilities selected for the exercise.
Validation:
(10) The validation module should indicate that all fields have been correctly filled in (with a green “OK”). If this is the case, you can hit the "Compute" button to launch the analysis. If this is not the case, the "Compute" button will still be in red and you will have to go through the warning and error message to find out what needs to be adjusted.
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Once this is done, go back to the Data module to check the eight six output datasets that have been generated:
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- scenario processed class: Table containing the travel scenario that has been processed.
- result geographic coverage analysis class: Table containing the results of the geographic coverage analysis.
- zonal coverage class: Table containing the results of the zonal statistics analysis in case this option has been checked.
- population on barriers class: Raster format layer containing the spatial distribution of the target population on barriers.
- population residual class: Raster format layer containing the spatial distribution of the residual population.
- catchment class: Vector format layer containing the extent of the catchment area for each facility.
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Next, just as you did above for the first part of the exercise, you now need to archive the results, export and unzip them in order to open and visualize the results. All of this can be done in the "Data" module (see Section 5.4).
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- cat: unique identifier of the health facility as per to the field selected from the attribute table of the health facility layer.
- name: name of the health facility as per the field selected from the attribute table of the health facility layer
- capacity: maximum coverage capacity of the health facility expressed as the number of people that the facility can serve (as per the field selected from the attribute table of the health facility layer).
- amRankValues_capacity: values for the parameter used to define the processing order. In the present exercise, this column contains the maximum coverage capacity of each facility as this is the option that was selected above. Please see the boxed note below for more information regarding the label and content of this column when using the other options.
- amRankComputed: processing order applied during the analysis based on the content of the "amRankValues_capacity" field and the direction of processing (ascending or descending), as selected by the user.
- amTravelTimeMax: maximum travel time in minutes, as set by the user for the analysis.
- amPopTravelTimeMax: total population located within the maximum travel time set for the analysis (amTravelTimeMax).
- amCorrPopTime: Pearson correlation coefficient between the set of travelling times (t, from 0 to the maximum travel time) and the corresponding covered population within this time step (i.e. the sum of population in all cells that are located between t and t+1 of travelling time). This correlation measure gives a rough estimate of how the population is distributed through space as we move outward from the health facility. As an example, a large positive value (e.g. 0.707 in the third line of the above table) means that the population is relatively uniformly distributed as you expand outwards from the facility. A strong negative correlation would mean that there is more population close to the health facility than there is far away from it. A correlation close to zero means that there is no specific tendency in how the population is spread within the catchment. This correlation should really just be used as a relative indicator, as we do not provide the statistical significance of the correlation (you should use proper GIS analysis outside AccessMod if you need statistical insights about how the population is spatially located within the catchments).
- amTravelTimeCatchment: travel time to reach the maximum extent of the catchment area attached to the health facility. This value is either:
- Equal to the maximum travel time set for the analysis (amTravelTimeMax) when the maximum coverage capacity of the facility has not been reached within the set time (120 minutes in this example). In the current exercise, this is the case for the Queen Elizabeth Hospital.
- Smaller than the maximum travel time set for the analysis (amTravelTimeMax) when the maximum coverage capacity of the facility is reached before reaching amTravelTimeMax. This is the case of The Medka Health Centre in the exercise.
- amPopCatchmentTotal: population located within the catchment area for the travel time reported in the amTravelTimeCatchment field
- amCapacityRealised: part of the maximum coverage capacity of the health facility that is being used (realized) based on the total population located in the catchment area for the given travel time. This value is:
- Equal to the maximum coverage capacity of the health facility (capacity) when this value is reached before the maximum travel time (amTravelTimeMax). An example is the Medka Health Centre in the present exercise.
- Smaller than the maximum coverage capacity of the health facility (capacity) when such capacity is not reached within the maximum travel time (amTravelTimeMax). An example is the Chiradzulu District Hospital is in this the present exercise.
- amCapacityResidual: part of the maximum coverage capacity of the health facility that is not being used. This value is calculated by making the difference between the maximum coverage capacity (capacity) and the realized capacity (amCapacityRealised). This value is therefore equal to 0 when the maximum coverage capacity is being reached within the given travel time.
- amPopCatchmentDiff: Part of the total population located in the final catchment area (amPopCatchmentTotal) which does not experience facility coverage by lack of sufficient coverage capacity in the health facility. This value is calculated by making the difference between the total population in the catchment area (amPopCatchmentTotal) and the realized capacity (amCapacityRealised). This value is therefore:
- Equal to 0 when the maximum coverage capacity is not reached within the given travel time
- Greater than 0 when the maximum coverage capacity is reached within the given travel time
- amPopCoveredPercent: cumulative geographic coverage expressed as a percentage. This value corresponds to the percentage of the initial population distribution grid (i.e. the layer selected in the "Select population layer (raster)" field) that has been covered after having processes the health facility in question.
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The header of the columns containing the unique identifier, the name, and the maximum coverage capacity of each health facility in the resulting table will be the same as the header of the field you have selected from the attribute table of the health facility layer. The spelling of the header and content of the column containing the values used for defining the processing order will be as follows:
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