USWFCA: Enhanced Two-Step Floating Catchment Area Accessibility Tool
Short Guide to Installation and Use
Background:
The USWFCA tool is an ArcGIS Desktop Add-In, developed using Visual Studio and the Microsoft .NET platform, and currently written in VisualBasic.NET by Dr Mitchel Langford (University of South Wales) and Dr Richard Fry (University of Swansea).
The purpose of this tool is to facilitate easy computation of so-called “Enhanced Two-Step Floating Catchment Area” (or E2SFCA) accessibility scores, together with some other commonly used geographical accessibility metrics.
It requires an ArcGIS Desktop installation (currently version 10.1 or 10.2) which includes the Network Analyst extension (see link below for more details)
http://www.esri.com/software/arcgis/extensions/networkanalyst
Using functions contained within this extension it computes the closest service supply point facility for each demand point, and generates an origin-destination matrix of the network travel costs between the designated service supply and demand points subject to a threshold distance/time parameter, from which the ES2FCA scores and other metrics are then computed.
Once installed, to operate the Add-In you will require access to an appropriately configured network dataset (see link below for more details)
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//004700000007000000
You also require two point feature classes (or shapefiles) that represent the locations and attributes of the service supply and service demand points respectively. These inputs may have a service supply capacity and demand volume identified within their attribute tables (for example, the number of beds available at each hospital, and the number of people living at each demand point). If such information is not available the set of points can be given a default weighting of “1” instead.
Both the supply and demand points must have had their network location (on the network dataset previously described) pre-computed using the Calculate Locations function as found under Network Analyst Tools in the ArcGIS toolbox…
Successful execution of this function adds additional attribute fields consisting of:
· SourceID: The numeric identifier of the source feature class that the network location is located on.
· SourceOID: The numeric identifier of the source feature.
· PosAlong: The position along the digitized direction of the source line feature. This value is stored as a ratio; or is null if the network location references a junction.
· SideOfEdge: The side of the edge in relation to the digitized direction of the line feature. This field is limited to two values: Right Side (1) and Left Side (2).
Installation:
The download will either be in the form of a single ESRI ArcGIS Desktop add-in file with the name USWFCAvXXX.esriAddIn (where XXX is a version number), or as four individual files with the same name and with extensions .dll, .esriAddIn, .pdb, and .xml. The USWFCAvXXX.esriAddIn file is actually only a zipped folder in which these four component files are contained.
To install:
Add-in files are automatically discovered and plugged into the desktop application at runtime if they are placed in a well-known local folder. For the Windows7 operating system this will be located at:
<Windows Installation Drive>:\Users\<your user name>\My Documents\ArcGIS\AddIns\Desktop10.x
(1) Using the USWFCAvXXX.esriAddIn file
Just double-click the file and the ESRI ArcGIS AddIn Installation Utility should start automatically and copy the contained files into the well-known folder…
Just click on the Install Add-In button, and if successful you should see…
(If the ESRI ArcGIS AddIn Installation Utility does not start automatically when you click the file, use any ZIP file management program to unzip the 4 files contained within it into the well-known folder or elsewhere and then proceed as described in method (2) below)
Start ArcMap and load your ArcMap MXD document, or otherwise configure it such that it contains the necessary datasets. From the menu select…
Switch to the Commands Tab and wait for it to finish scanning. Locate the Add-In Controls entry and drag the FCA button onto any toolbar of your choice…
To start the tool, simply click on the E2SFCA tool button…
(2) Using the four individual USWFCAvXXX files
Copy the files to any folder of your choice (this includes the previously mentioned well-known local folder, where the files will be automatically discovered and plugged into the desktop application at runtime)
If you decided not to place them into the well-known folder, start ArcMap and from the menu select
In the Add-In Manager switch to the Options tab.
Add the folder in which the files are now stored. When you switch back to the Add-Ins tab the FCA tool should now show up as being installed…
Click the Customize button. Switch to the Commands Tab and wait for it to finish scanning. Locate the Add-In Controls entry and drag the FCA button onto any toolbar of your choice…
To start the tool, simply click on the E2SFCA tool button…
Using the Tool (brief instructions):
To start the tool, click the E2SFCA tool button…
If your current map document does not contain any network dataset you will first see an error message…
there is little point continuing until you resolve the issue (i.e. you create/load a suitable network dataset).
The first form of the E2SFCA tool is now displayed…
The first network dataset listed in your table of contents will be pre-selected in the upper drop-down list. If there is more than one network dataset available you can select an alternative by using this drop-down.
With a network dataset selected the second drop-down displays all the travel impedance/cost fields which it contains. Typically this will include a distance metric (e.g. Length) but you may have configured your network dataset to contain other cost metrics, such as travel time…
The units associated with the travel impedance/cost field selected will be displayed in the third panel where the FCA threshold parameter is to be set.
For example, with a length impedance field set this might be reported as
whilst with a time impedance field set this might be reported as
In the third panel you set the Floating Catchment Area threshold. Using a distance cost metric this will be a travel distance, using a time cost metric this will be a travel time. So, for example, you might specify 5000 metres, or maybe 15 minutes travel time. Be aware this is a particularly critical parameter in the FCA calculation. All demand points within the threshold distance/time will contribute to the Step 1 score for each supply point; and all supply points within the threshold distance/time will contribute to the Step 2 FCA accessibility score for each demand point!
FCA scores are ultimately a supply to demand ratio (for example, the number of doctors accessible per person able to reach them). These scores are typically quite small; the final drop-down allows for a scaling factor to be applied which can help to preserve precision and make the numbers less unwieldy.
Setting 1000, for example, simply multiples any final FCA scores by this factor, or moves the decimal point 3 places to the right if you prefer to think of it that way.
Pressing the button moves on to the second form…
The upper drop-down lists all point feature class layers in your current MXD document (these should not be placed inside Group Layers). Use the list to select which layer represents the locations of Supply Points on your network dataset. As mentioned before, this layer should have the network locations of these points pre-computed using the Calculate Locations function of the Network Analyst toolbox and using the Network Dataset that has just been specified for use on form 1.
Use this checkbox if you wish to use an attribute field in this layer to represent the service supply capacity (e.g. number of doctors, number of hospital beds, number of tennis courts, number of busses at a bus stop, etc.). Then select the field name from the drop-down list which becomes activated (it will only show suitable numeric field types). If this item is left unchecked a default supply capacity value of “1” will be assumed.
Pressing the button moves on to the third form…
This is very similar to the previous form, allowing you to configure the layer containing the service demand points. This will typically be a set of points that hold some form of population count – hence the demand capacity option is set on by default and the drop-down list is set to the first numeric attribute field. Be careful to reset this to the correct field if your population count is not the first numeric field, or if an alternative measure is appropriate. You can disable the option altogether too if that is more appropriate, in which case a default demand volume of “1” will be assumed at each demand point.
Pressing the button moves on to the forth form…
This focuses on the specification of any distance-decay function. The original Floating Catchment Analysis (FCA) method had no distance decay function – but the Enhanced FCA is so-called because it defines a decay function such that people become less inclined to utilise a service as their distance to it increases, until the threshold distance/time (as set on form 1) is reached, at which point the demand falls to zero.
The classic option is thus the original FCA method. The others offer alternative models of the rate of decline with distance. Linear simply tapers off uniformly down to the threshold point, while the Gaussian and Butterworth options offer a slower initial decline followed by a faster rate thereafter. The graphics help to illustrate the general effect. Both require parameters to specify the detailed shape of the function. In the Gaussian model a figure around 50 is typical. Higher values cause a steeper rate of decline in the middle section, lower values produce a gentler curve closer to the linear model. In the Butterworth model value of 6 and 50 are good starting points. Increasing the power function steepens the declining section, while increasing the passband value extends the flat shoulder so it extends further to the limit of the threshold.
The final option here is to specify the folder in which the dbf results tables containing E2SFCA scores will be located.
Pressing the button moves on to the fifth form…
You are ready to compute the scores – just press the tick button to start. Depending on the amount of data to process, the parameters set, and the hardware used, the computation may take some time. You should see Closest Facility and OD Matrix network analysis layers added to the bottom of the Table of Contents as computation progresses. Messages arising from the actions of the Network Analyst operations may appear in the left-hand list box.
Typically these report back if it has not been possible to compute a nearest distance pathway or an Origin-Destination cost to of any of the supplied points (see the ArcGIS documentation for further information on how and why this might arise and how to deal with the issue).
On completion of the calculations you will be informed of the table name in which the results have been saved. You also have the option of removing or retaining the network analysis layers on which the calculations are based when the E2SFCA tool is closed.
Pressing the button completes the task and closes the tool.
Interpretation of Results:
The results are presented in a normal dbf data file…
The DemandID field will match to the FID codes of the points in the specified Demand points layer. The SupplyID will match to the FID codes of points in the specified Supply points layer (i.e. identifies which supply point is the nearest to each demand point through the network). The SupplyDist field reports the actual distance/cost value. The Choices field reports how many supply points in total lie within the threshold FCA limit you set. The MeanCost field reports the average distance to all supply points that lie within the threshold FCA limit set. And the E2SFCA field reports the Two-Step Floating Catchment Area accessibility score using the options (i.e. threshold distance and distance decay model) you specified.
It is possible to get a score of 0 for E2SFCA, MeanCost, and Choices metrics - this is perfectly logical and simply indicates that there are no supply points within the threshold distance that was specified (this can be confirmed by reference to the SupplyDist value which should always exceed the value set).
As a checksum on the operation of the FCA calculation, it can be noted that if the E2SFCA score of each Demand point is multiplied by the demand volume (e.g. population count, or just “1” if nothing was specified) and summed across all points – this total will match the total supply volume in the system. This will simply be the number of supply points if “1” was used as the supply volume, or is the total number of hospital beds, number of doctors, etc., in the system (note if a pathway to any supply points was not found, as reported back on form 5, these points and their supply volumes are excluded from this total).
Mitch Langford April’14