Problem of calculating distance between Point Geometries.

[Jae_Jeon]

Hello,

I passed the same distance as a parameter into two different methods of the same Point Geometry, Buffer() and pointFromAngleAndDistance(), but the results shows different distance. 

a. Point Geometry / b. point from the method pointFromAngleAndDistance() / c. buffered polygon from Buffer()

1. distance from pointFromAngleAndDistance()  /  2. distance(radius) from Buffer()

The spatial reference of Point Geometry is EPSG 4326. (The point geometry objects for two methods is the same so, those methods are assumed to use the same distance measurement units of the same geometry's spatial reference.)

I also add a line of code to make the process.

My question is how to match two distances(1,2 in the image above).

Thanks for any help. 

[DanPatterson]

it is the coordinate system of your map. probably web mercator? if so, don't use web mercator to do any distance and direction measure, use a compromise (eg UTM ) or equadistant coordinate system

[curtvprice]

Dan, if you look at the code, @Jae_Jeon  is using geodesic distance, which avoids the coordinate system issue, but something weird is happening and it appears the buffer is not the size he's looking for.

On reading the help for arcpy.PointGeometry I'm wondering if what you need to use is not the default GEODESIC but, instead, GREAT_ELLIPTIC for your distance-angle method.

[Jae_Jeon]

Thanks for your answer. 

I tested it with 'GREAT_ELLIPTIC' and the result was not differnet from the one from GEODESIC. 

I also checked the output buffer geometry's spatial reference follows point geometry(epsg: 4326) 

[DanPatterson · Accepted solution]

I still think it is a display thing since a buffer can only create one radius,  I am thinking a Tissot's Indicatrix related issue since the SR is 4326

[curtvprice]

Oh I get it. Change the CS of the map to equidistant and the point may line up with the buffer better.

[Jae_Jeon]

It works properly after I changed SR to both UTM and equidistant one. 

Thanks for your help! 

[Anonymous User]

just to confirm: PointGeometry.buffer() creates a planar buffer (circle in the flat plane defined by the spatial reference), not a geodesic one.

For correct distance measurement at this map scale, I recommend using either the Buffer geoprocessing tool with geodesic method, or repeated application of pointFromAngleAndDistance with the geodesic method.

Jim TenBrink

spatial analyst team

[Jae_Jeon]

It's good to know. 

Thanks Jim.