topic Question on adding Azmuth/Direction value to a line in ArcGIS Spatial Analyst Questions

Question on adding Azmuth/Direction value to a line

JeffChartrand — Wed, 16 May 2018 12:54:09 GMT

Confused: I created a viewshed using Azimuth (140) and 2 (220) to limit viewing angle. To add clarity I add a line in create features using direction and distance. I select the point feature to start the line and right mouse click to bring up drawing options. I select the Direction/Length option (140, 1500) which draws a line. I repeat the process for the second line (220,1500). They do not match the viewshed azimuth and when I add geometry to the line to add values in attribute table the direction (Azimuth) values do not match those imputed in create feature.

The line created with direction140 becomes 230

The line created with direction 220 becomes 310

How do I get all the Azimuth values to agree?

Image attached to show displacement.

Any and all help appreciated.

Jeff

Re: Question on adding Azmuth/Direction value to a line

RobertBorchert — Wed, 16 May 2018 16:29:23 GMT

What direction do you think is Zero?

In ArcMAP the system defaults to Arithmetic directions. So Zero is EAST and it rotates counterclockwise as you go up. So North is 90, west is 180, south is 270

You will notice that your directions are off by 90 degrees

Re: Question on adding Azmuth/Direction value to a line

JeffChartrand — Thu, 17 May 2018 08:08:50 GMT

Hi, I know for some tools it is 0 to east but that does not explain why different tools in ArcGIS seem to produce different azimuth directions. The ESRI help for Viewshed Azimuth indicates 0 at north. Nor why when you use distance direction it seems to use the reciprocal value. And to make it even more interesting when I use add geometry tool to add line bearing it comes up with a different value 140 becomes 310. In looking at the values it would seem that the viewshed azimuth used North as 0 and goes clockwise, and line direction and bearing geometry has East as 0 and goes counter clockwise. Surely having two tools in ArcGIS using different approaches to azimuth is a bug that needs fixing?

Re: Question on adding Azmuth/Direction value to a line

DanPatterson_Retired — Thu, 17 May 2018 09:00:14 GMT

Azimuth and bearing and angle are sometimes ill-defined, and may be discipline specific.

I always check via calculation. Angles in math are formed between two lines.

When you-fill-in-the-name is calculated relative to the X-Axis they range between 180 to -180 if going in a clockwise direction (ie W, N, E, S). When relative to the Y-axis, going clockwise (ie N, E, S, W) they range from 0 to 360. Then there are variants (sun angles on the horizon for instance) where S is given 0 and E -90, N +/- 180 and W 90

Don't bother debating 'true' definitions, I have seem the terms used/abused countless times, hence, my choice of confirming via calculation