topic Counting points in buffers in Data Management Questions

Counting points in buffers

ErikaMartino — Sun, 25 Sep 2016 08:10:46 GMT

Hi All,

Up until now, everyone on this forum has saved my bacon, so I thougt I would try my luck again! I am counting facilites (points) within buffers using spatial joing and sum, the problem, however, is this: when I do a 'blind eye' count, the count is 2, for example, in buffer A yet when I look at the spatial join count it shows as 10! Could someone suggest why this would occur? The points are listed as "multipoints" in the geography type, would this distort the count and if yes, how do I rectify this issue?

Thanking you in advance!

Re: Counting points in buffers

JayantaPoddar — Sun, 25 Sep 2016 08:42:36 GMT

The multipoint geometry could be causing the issue. The geometry should be "point".

Use the following steps to convert the above data to point feature class.

1. Add two new fields in the attribute table, viz., "X" and "Y". Calculate the respective X and Y coordinates using CALCULATE GEOMETRY. Choose "Decimal Degrees" as units.

2. Using Table to Excel tool, convert the table to Excel.

3. Open the excel file in MS Excel. Delete the Geometry field.

4. In ArcMap, File > Add Data > Add XY data. Use appropriate spatial reference (e.g. GCS_WGS_1984 or any other appropriate GCS).

5. Export the Event layer to a Feature class/shapefile.

Then try the Spatial Join once again.

Re: Counting points in buffers

DanPatterson_Retired — Sun, 25 Sep 2016 11:52:04 GMT

blow the file apart using Multipart To Singlepart—Help | ArcGIS for Desktop

things are never as they appear, so don't rely on your eyes... always check the geometry

Re: Counting points in buffers

ErikaMartino — Sun, 25 Sep 2016 22:35:51 GMT

Thank you both, there goes a week of analysis, at least now I know what the issue was!

Re: Counting points in buffers

ErikaMartino — Sun, 25 Sep 2016 22:38:23 GMT

Also, would this method also work;

How To: Convert a multipoint feature class into a point feature class ?

Re: Counting points in buffers

ErikaMartino — Sun, 25 Sep 2016 23:03:54 GMT

Hi All,

One last question: I am using Network Analyst to find the nearest distance to a park from a resident's location (i have converted the park polygons to line and then vertices). I keep getting this error? Could someone suggest an alternate?

Re: Counting points in buffers

DanPatterson_Retired — Sun, 25 Sep 2016 23:06:39 GMT

why not use the builtin tool? are you using an old version of arcmap (ie 9.3 or less as per the link you sent)

Re: Counting points in buffers

DanPatterson_Retired — Sun, 25 Sep 2016 23:08:39 GMT

it is referring to the road network not the facilities you should have had a shape field in the FC

Re: Counting points in buffers

ErikaMartino — Mon, 26 Sep 2016 01:17:04 GMT

Hi Dan,

Thanks for prompt response - I have setup a network dataset (shape file) that has streets and junctions, however, when I go into the properties toolbar to change the attributes of the network the only attribute listed is length, with no time component. Could you offer advice on how I might fix this? The XY are the resident locations, the greenspace file is the vertices of the green space, however, the network dataset is coming up as an error? The distance is the length, but why do I need to assign time to this?

Re: Counting points in buffers

DanPatterson_Retired — Mon, 26 Sep 2016 01:48:57 GMT

the simplest way is to add a field and calculate time from an assumed speed of traversal.

Consider the simplest idea, that the speed 'limit' is 50 km/hour. You can calculate the time of traversal for a segment of a 'shape' from its length. So if you have a 1 km road, how long do you think it would take you in minutes or seconds to traverse that road segment? You can categorize your mode of transportation and/or road (read, network edges) according to a 'limit'. This obviously explains why the shortest path need not be the fastest since various routes to a destination from an origin can have different physical lengths and a 'temporal length'.

So you will have to add this constraint into your analysis, either keeping it simple, or categorizing your 'roads/edges/routes/paths' accordingly.

Re: Counting points in buffers

ErikaMartino — Mon, 26 Sep 2016 02:27:39 GMT

Hi Dan,

Thank you as always for speedy reply. Could you detail the step by step process I would need to undertake to do this? I am not per se interested in speed / time, however, I have googled the process and tried to add time but the analysis keeps failing, see below, I have added parameters and evaluators (at least I think I have!) and still not working. Any advice would be very helpful. Neither data inputs have a time table, which I have tried to add using field calculator but it won't give me that option...

Re: Counting points in buffers

DanPatterson_Retired — Mon, 26 Sep 2016 02:53:58 GMT

You missed my point... you have to add a time field to your table, and calculate time from the Shape length (aka distance) and some 'speed limit'. if you have distance, distance/unit time you can calculate time.

Re: Counting points in buffers

ErikaMartino — Mon, 26 Sep 2016 03:51:49 GMT

Hi Dan,

I was able to add a time field in my table but I can't use field calculator to set a time, see blue... (eg. (length/mph*60)). I can't input that formula for some reason...

Re: Counting points in buffers

ErikaMartino — Mon, 26 Sep 2016 04:42:02 GMT

Hi Dan,

I set up a new network dataset and was able to apply a speed limit of 60p.h, however, when I run find nearest facility, still no output (see left column)

Re: Counting points in buffers

DanPatterson_Retired — Mon, 26 Sep 2016 08:03:41 GMT

not seeing a proper road network... the map isn't import right now.... did you calculate the required values in a field and use that field as suggested.... this is getting off topic as well. You should branch this whole network issue, since the thread and its title have diverged some time ago