topic Re: Help: Drawing straight lines using the polygon tool in ArcGIS Mapping and Charting Questions

Help: Drawing straight lines using the polygon tool

JoshuaLeigh — Thu, 26 Apr 2018 16:50:34 GMT

Hi,

I'm currently using Arcmap 10.3.1. and I am trying to manually outline glacier extent on Landsat imagery at 30 m resolution. I wan to draw a straight line around each pixel edge to create a polygon of glacier area. When I am drawing the line they often have angle changes in them so that they do not appear straight (see image below).

Is there a way to efficiently draw perfectly straight lines? - e.g. shift and drag as in Adobe Illustrator to get a perfectly straight line

Thanks in advance.

Re: Help: Drawing straight lines using the polygon tool

JosephCarter — Thu, 26 Apr 2018 18:42:07 GMT

Hi, Josh. Your best bet is the GOGO toolbar.

Good luck!,

Joe

Re: Help: Drawing straight lines using the polygon tool

AbdullahAnter — Fri, 27 Apr 2018 16:09:36 GMT

I notice that you are to digitize the pixels of raster that have the four primary direction ( 0 - 90 - 180 - 270 )

so the Right angle tool will be appropriate or this purpose

Re: Help: Drawing straight lines using the polygon tool

DarrenWiens2 — Fri, 27 Apr 2018 16:16:59 GMT

Maybe it's more complicated than this, but have you ruled out choosing some raster value threshold, setting values above to 1, below to 0, and creating a polygon from that mask? At the very least, I'd try to use this as a starting point - digitizing raster cells looks torturous.

Re: Help: Drawing straight lines using the polygon tool

JoshuaLeigh — Fri, 27 Apr 2018 17:41:49 GMT

Thank you for suggesting this. I have just tried this method out and it performs the task in a much quicker and more accurate way than I had managed previously.

Re: Help: Drawing straight lines using the polygon tool

JoshuaLeigh — Fri, 27 Apr 2018 17:42:43 GMT

Thank you, I will have a read up on how to use this tool.

Re: Help: Drawing straight lines using the polygon tool

JoshuaLeigh — Fri, 27 Apr 2018 17:44:36 GMT

Thank you for your suggestion. I am currently trying to assess how accurately a automated thresholding process generates glacier outlines and compare this to a manual digitisation. Hence the need for a quick and reliable way to digitise in straight lines.

Re: Help: Drawing straight lines using the polygon tool

DarrenWiens2 — Fri, 27 Apr 2018 17:59:36 GMT

Okay, counter-proposal: digitize freehand identifying the raster cells to be considered the "manual digitization", convert to raster and back out to line/polygon to get the straight lines.