Near feature and selection feature

[MadiBenson]

Hi,

I am using Arc GIS to map the location of events and the locations of schools in a region. I have then used the programme to find the number of events occurring within 300meters of a school. 

I have utilised the near tool for this as well as the select by distance from location method. These two tools are giving me different answers as to the number of events occurring within 300meters of a school.

Does anyone know why that would be?

Thanks,

Madi 

[WillHughes2]

Are you using ArcGIS Maps for Office or ArcMap or Pro? What difference are you seeing in the results? 

[MadiBenson]

Hi Will,

I am using ArcGIS Maps for Office 

The results have been calculated for events occurring within a certain distance of a school i.e.. 100m, 200m, 300m etc. up to 1000m.

For example:

Using the near tool I get 

100m - 425 events

200m - 2093 events

300m - 4250 events

Using the selection method

100m - 234 events

200m - 1284 events

300m - 2797 events 

Looking at the data the discrepancy in the results cannot be explained by one event being counted as being within a certain radius of more than two schools. 

[WillHughes2]

Can you explain the workflow for the selection method?

[AmyGwin2]

Madi,

Sounds interesting. I have never used ArcMap for Office, but I use the other ESRI software.

If the selection is made from a point, it should be straightforward. If the selection is made from a polygon, then it could be measuring from the centroid of the polygon, or the outside of the perimeter of the polygon. Are your schools in your task points?

see more information on esri's 'near': Near—Help | ArcGIS Desktop 

By the way if you want to find the "concentrations" of a set of events, you would use 'Mean Center'.  Mean Center—Help | ArcGIS for Desktop 

Hope that helps.

[MadiBenson]

Hi Will,

Have outlined the methods below.

Selection method:

Menu

Selection

Select by location

Picked source layer and target layer

Are within a distance of the source layer feature (distance 100m, 200m, 300m etc.)

Near tool method:

Toolbar

Near tool

Location

Source layer and target layer

Within x metres (100m, 200m, 300m)

Geodesic distances

Thanks,

Madi

[WillHughes2]

Ok, so it looks like the selection method is being done in ArcMap. What is the spatial reference of your data and the data source - hosted feature services? Can you provide a copy of your data? I suspect the difference is due to planar vs geodesic distances. Use the buffer tool in ArcMap and compare the results. 

[MadiBenson]

Hi Will,

I used the WGS 1984 in each. The data source was an excel table of latitude and longitude data for individual addressed. Unfortunately I cannot provide a copy of the data due to the nature of the events. I did select geodesic where there was an option to select that. Is there a way to know which is the most accurate method?

Thanks,

Madi

[WillHughes2]

The link below describes planar (euclidian) vs geodesic. You should use planar for projected data and geodesic for data in a geographic coordinate system. 

Buffer—Help | ArcGIS for Desktop 

I'd suggest that you call in a case with Esri Support. 

https://support.esri.com/en/contact-tech-support