[GIS] Different results in OD Cost Matrix analysis and Route analysis

arcgis-desktopnetwork-analystrouting

I am using OD Cost Matrix layer in order to find distances between many origins and destinations points. It works fine, but I want to make sure the results make sense, and since OD Cost Matrix can only produce straight lines (and not true route shapes), I tried to calculate some specific origin-destination pairs using a Network Analyst regular Route layer and compare them to the OD Cost Matrix results.

However, in some of the cases the distances seem to be different between the two methods, even when I use the exact same analysis settings and network restrictions. Sometimes it's only a small gap, but in other cases it's probably a whole different route.

Shouldn't the two tools produce the exact same routes?

Best Answer

So here are the code and summary of the aforementioned bugs, as requested by @Chris W. Basically they seem to refer to the same issue and to have the same cause, i.e. a problem with the OD cost algorithm.

NIM094092 - The OD cost matrix analysis in ArcMap returns different travel time results for one origin if another origin is present.
NIM060948 - Time and length returned from solving OD Cost Matrix problem varies between the same origin and destination depending on the number and location of input origin points.
BUG-000084762 - When solving an OD Cost Matrix for a large network dataset using Hierarchy, erroneous results are returned in a small number of combinations where the route connecting an Origin and a Destination is of a longer duration than any other route from the same Origin and Destination that diverts through a third location.

They've been found between versions 10.0 and 10.2.2 and are all assigned.

As I commented before, an alternative could be to use Closest facility analysis (in an automated process) to get a similar output and see if the results are more reliable. I suppose the feasibility of this depends largely of the number of origin and destination points.

Related Solutions

[GIS] Creating an Origin/Destination (OD) cost matrix using ArcPy

I would put the core of your code within a "for" loop. Something like this:

for year in range(1999,2005): #python doesn't include 2005 here and will only go up to 2004
  outNALayer_Year = 'outNALayer'+str(year)
  Pas_Lyr_Year = 'Pas_Lyr'+str(year)

  arcpy.MakeFeatureLayer_management (inOrgins, Pas_Lyr_Year)
  arcpy.SelectLayerByAttribute_management (Pas_Lyr_Year, "NEW_SELECTION", " f_aar = str(year) ")

  #Create a new OD Cost matrix layer.
  arcpy.MakeODCostMatrixLayer_na(inNetworkDataset, outNALayer_Year, "Minutes", "", "", "Minutes") 

  #Load the locations as origins.
  arcpy.AddLocations_na(outNALayer_Year, "Origins", "Pas_Lyr_Year", "Name f_aar #","1000 Meters", "SYKEHUS")

  #Load the locations as destinations 
  arcpy.AddLocations_na (outNALayer_Year, "Destinations", inDestinations, "Name SYKEHUS #","1000 Meters", "SYKEHUS")

  #Solve the OD cost matrix layer
  arcpy.Solve_na(outNALayer_Year)

  #Save the solved OD cost matrix layer as a layer file on disk with relative paths
  arcpy.SaveToLayerFile_management(outNALayer_Year, outLayerFile_Year,"RELATIVE")

Anytime you reference outNALayer_1999, outNALayer_2000 and so on, replace it with outNALayer_Year. Same goes for Pas_Lyr1999, etc.

Note, I haven't tested this, but that's the logic I would use.

[GIS] How to create an OD road distance matrix in QGIS

UPDATE - QGIS3:

Since this seems to be an important topic, I would like to share my answer on a similar question.

If you are using the current version of QGIS (3), you can use the plugin QNEAT3. It offers multiple processing algorithms that produce origin-destination matrices (OD-Matrix) as line layer, table or csv file out of the box. All algorithms rely on the dijkstra() method in the qgis.analysis module, therefore all costs are calculated on the basis of shortest paths and make use of C++ code for faster execution.

You can get more information about the plugin at the qgis plugin repository and at the plugins documentation.

Best Answer

Related Solutions

[GIS] Creating an Origin/Destination (OD) cost matrix using ArcPy

[GIS] How to create an OD road distance matrix in QGIS

Related Question