topic Re: Point Density Question in ArcGIS Spatial Analyst Questions

Point Density Question

ThomasMelton1 — Tue, 21 Feb 2012 16:30:03 GMT

I am doing a point density analysis on about 1500 points across southern California. The tool and results are working correctly, but value's assigned to them do not make sense.
Under the Table of Contents the Values are

0-20,600
20,700-41,200
41,300-61,900
62,000-82,500

...and so on, going in 20,000 increments for 10 values. How do I make sense of these numbers for the general public such as "orange means there is a density of 15 people and blue means there is a density of 45 people"
Or perhaps this is the wrong extension I am using?

Thank you for your help.

Re: Point Density Question

ThomasMelton1 — Tue, 21 Feb 2012 19:42:59 GMT

Can anyone help me? Thank you

Re: Point Density Question

ThomasMelton1 — Tue, 21 Feb 2012 20:40:46 GMT

From what I can tell if I dived my number by 250 it is the desired number I'm looking at. Is this because the Output Cell Size is not set?

Re: Point Density Question

EricRice — Tue, 21 Feb 2012 21:57:27 GMT

Greetings,

Point Density provides a count per unit area. To understand the output you have to understand the units of the input and the parameter called, "area_unit_scale_factor". The tool goes to a cell, looks for points in the search radius and sums them, then divides by the area of the search radius. For circular or rectangular its quite easy to confirm the results by hand.

For more examples about understanding the output units, please review, How Point Density works.

What are the units of the points, and what did you fill in for all the other parameter values?

Regards,
Eric

Re: Point Density Question

DavidVon_Stroh — Thu, 08 Mar 2012 21:50:19 GMT

I am having trouble myself with point density. I have a shapefile of 45000 points spread across the city of Seattle. I want to create a density map of these points. Everytime I try to use the point density tool though, it fails. Advice?

David

Re: Point Density Question

TarunJaiswal — Thu, 08 Mar 2012 21:57:14 GMT

I am having trouble myself with point density. I have a shapefile of 45000 points spread across the city of Seattle. I want to create a density map of these points. Everytime I try to use the point density tool though, it fails. Advice?

David

Hi David,

Do you get any error message, when Point Density tool fails?
If you are using ArcGIS 10 you can use the Results window to find it.

Thank you,

Tarun

Re: Point Density Question

deleted-user-IR249IovB3CN — Wed, 20 Jun 2012 15:16:42 GMT

I am also trying to make sense of the density tools. Here are my settings. Spatial units = meters. Output cell size = 1000m, Neighborhood is circle of 50,000m, Area Units = Square Kilometers. The numbers in the output seem very low ranging from 0.2-7.2. The file consists of over 400,000 points. Are these values # of points/km2 within each neighborhood centered on analysis cell?

For example, if 2000 points occur in the neighborhood then for the output cell (or analysis cell) the value of that cell will be # of points/neighborhood area(in area units defined here as square Km) or 2000/7853980 Km2 = 0.0002546 pts/km2? Is that correct?

Re: Point Density Question

EricRice — Wed, 20 Jun 2012 17:59:50 GMT

I am also trying to make sense of the density tools. Here are my settings. Spatial units = meters. Output cell size = 1000m, Neighborhood is circle of 50,000m, Area Units = Square Kilometers. The numbers in the output seem very low ranging from 0.2-7.2. The file consists of over 400,000 points. Are these values # of points/km2 within each neighborhood centered on analysis cell?

For example, if 2000 points occur in the neighborhood then for the output cell (or analysis cell) the value of that cell will be # of points/neighborhood area(in area units defined here as square Km) or 2000/7853980 Km2 = 0.0002546 pts/km2? Is that correct?

Matt,

The density values are calculated for each cell. The cell center is where the neighborhood begins. Your example came to the wrong answer. Attached is how you would obtain something that fits in your range of 0.2 -7.2 using your example of 2000 points occurring in a neighborhood that size.

Best,
Eric

Re: Point Density Question

deleted-user-IR249IovB3CN — Wed, 20 Jun 2012 19:03:09 GMT

Thank you Eric. That makes sense. I need to return to the 4th grade math texts! I was thinking linear units not areal units! So the output is the number of points per square kilometer within that cells neighborhood. Sounds so simple now.

Re: Point Density Question

Anonymous User — Thu, 02 Feb 2017 17:46:13 GMT

erice-esristaff‌ the image upload is corrupt and does not load, it's cut off part way through.