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Regarding creation of Kernel Density Map in ArcGIS

Question asked by naqvihm on Jun 19, 2015
Latest reply on Jun 22, 2015 by xander_bakker

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I wish to plot Kernel Density Map for geo-referenced rural road crashes in ArcMap 10 (using Spatial Analyst Tools->Density->Kernel Density-Dialogue box pops up to enter data). I found that the "Output Cell size" and "Search Radius" by default the ArcGIS software is taking for plotting Kernel density Map for road crashes is very small say 0.001 metre(m) and 0.01 m respectively, which is small to conclude inferences from the plotted kernel density maps. When I change"output cell size" and "radius" in the Dialogue box to larger values say 5 m and 50 m respectively, ArcGIS does NOT plot the kernel density map and show error. Could anyone help me to diagnose the problem in plotting kernel density maps for road crashes. Secondly I also request the esteemed members to write steps or procedure to carry out Network Density Estimation using ArcGIS as Kernel Density estimation is planar based and not popular in analysing highway crashes.

Thanking you in advance.

Reply to the cited query may be posted at my email account: naqvihm@gmail.com

H. M. Naqvi

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GSCUser85

Jun 19, 2015 12:15 AM

What coordinate system is your data in? Sounds like there is a mismatch between the coordinate system (geographic degrees perhaps) and you distance. 50 in degrees is a long way.

Probably need to get the data into a projected coordinate system first.

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naqvihm @ GSCUser85 on Jun 19, 2015 5:16 AM

Thanks for prompt reply. My crash data (latitude and longitude) are in degree and decimal. I used GCS->World->WGS43.Similarly for projected coordinate system is also taken as Northern hemisphere->WGS43. While making kernel density one is required to give search radius and Output cell size, these two items are to be given in metres or kilometres etc. May I request you to send your valuable inputs on the same.

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Dan_Patterson

@ naqvihm on Jun 20, 2015 2:32 AM

There is no one fixed equivalent. You have to project your data to a common projected coordinate system, not a geographic coordinate system

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xander_bakker @ naqvihm on Jun 20, 2015 3:10 PM

From the Help: default cell size is the shorter of the width or height of the output extent in the output spatial reference, divided by 250.

If ArcMap suggest the size 0.001, then the height or width of you data extent (smallest of the two) will be 0.25, which most certainly is expressed in decimal degrees.

...and it also states "Units are based on the linear unit of the projection of the output spatial reference". For these types of tools it is always best to work with a projected coordinate system as suggested by Neil Ayres and Dan Patterson .

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naqvihm @ xander_bakker on Jun 21, 2015 10:10 PM

Thanks for furnishing reply on cell size of kernel density map. My next question is as follows:

I have to analyse traffic accident data in order to investigate the density of crashes(point events). A standard (planar) Kernel Density Estimation (KDE) produce a smooth density surface of spatial point events over a 2-D geographic space. However, the planar KDE may not be suited for analysis road crashes (events), which usually occur inside a linear space.

Does anyone have suggestion how to plot network kernel density estimation for traffic accident data using ArcGIS tools?

Thanks in advance.

email: naqvihm@gmail.com

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xander_bakker @ naqvihm on Jun 22, 2015 5:50 AM

Kernel Density is an interpolation that does not have the option to include barriers. So in case of two parallel roads or two highways that cross at different Z altitudes, points located on one road will contribute to the density of the other. In this case it might be better to split up your network in sections and crossings and aggregate the number of accidents per part.

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6 Replies

GSCUser85 Jun 19, 2015 12:15 AM
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What coordinate system is your data in? Sounds like there is a mismatch between the coordinate system (geographic degrees perhaps) and you distance. 50 in degrees is a long way.
Probably need to get the data into a projected coordinate system first.
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- naqvihm @ GSCUser85 on Jun 19, 2015 5:16 AM
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  Thanks for prompt reply. My crash data (latitude and longitude) are in degree and decimal. I used GCS->World->WGS43.Similarly for projected coordinate system is also taken as Northern hemisphere->WGS43. While making kernel density one is required to give search radius and Output cell size, these two items are to be given in metres or kilometres etc. May I request you to send your valuable inputs on the same.
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  - Dan_Patterson @ naqvihm on Jun 20, 2015 2:32 AM
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    There is no one fixed equivalent. You have to project your data to a common projected coordinate system, not a geographic coordinate system
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  - xander_bakker @ naqvihm on Jun 20, 2015 3:10 PM
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    From the Help: default cell size is the shorter of the width or height of the output extent in the output spatial reference, divided by 250.
    
    If ArcMap suggest the size 0.001, then the height or width of you data extent (smallest of the two) will be 0.25, which most certainly is expressed in decimal degrees.
    
    ...and it also states "Units are based on the linear unit of the projection of the output spatial reference". For these types of tools it is always best to work with a projected coordinate system as suggested by Neil Ayres and Dan Patterson .
    1 person found this helpful
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    - naqvihm @ xander_bakker on Jun 21, 2015 10:10 PM
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      Thanks for furnishing reply on cell size of kernel density map. My next question is as follows:
      
      I have to analyse traffic accident data in order to investigate the density of crashes(point events). A standard (planar) Kernel Density Estimation (KDE) produce a smooth density surface of spatial point events over a 2-D geographic space. However, the planar KDE may not be suited for analysis road crashes (events), which usually occur inside a linear space.
      
      Does anyone have suggestion how to plot network kernel density estimation for traffic accident data using ArcGIS tools?
      
      Thanks in advance.
      
      email: naqvihm@gmail.com
      
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      - xander_bakker @ naqvihm on Jun 22, 2015 5:50 AM
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        Kernel Density is an interpolation that does not have the option to include barriers. So in case of two parallel roads or two highways that cross at different Z altitudes, points located on one road will contribute to the density of the other. In this case it might be better to split up your network in sections and crossings and aggregate the number of accidents per part.
        1 person found this helpful
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