topic How to create polygons from a raster by using specific RGB values in New to GIS Questions

How to create polygons from a raster by using specific RGB values

DaniBertram — Sun, 10 Feb 2019 20:41:19 GMT

Hi everyone,

I would like to create polygons, from a hardcopy map, which encloses the yellow, orange and red areas on the (hardcopy) map. (Please see attached 'NZ-CommercialFishing-AllMethods-A4-2.tiff'.) My approach so far has been to convert the map from a pdf to tiff file format, georeferencing and then manually creating the polygons. Manually creating these polygons is taking a really long time to do (two weeks and counting) and was hoping there was a quicker way of going about it. I'm considering cells with RGB values that meet the following criteria to be enclosed in a polygon: R > 200 && R > = G. (I'm using ArcMap 10.4.1 desktop and have access to quite a number of extensions.)

Any help would be greatly appreciated!

Re: How to create polygons from a raster by using specific RGB values

XanderBakker — Sun, 10 Feb 2019 21:36:14 GMT

Since the original information is clearly available in a GIS format (I assume already in some ArcGIS format) I wonder if you could contact the Fisheries Data Manager, Fisheries Management, Ministry for Primary Industries to get access to the data, before you start to derive the data from this map.

If that is not possible you could use image classification methods to create the areas, but it will never be as good as the original data.

Re: How to create polygons from a raster by using specific RGB values

DaniBertram — Mon, 11 Feb 2019 19:23:46 GMT

Hi Xander,

Thanks for taking the time to reply. I have emailed but have had no response, so I decided to go ahead and create my own shapefile.

I'm assuming that the image classification method that you are referring to is part of the Spatial Analyst extension? (If so I checked, and I do have that extension). Would it be possible for you to elaborate further on how I can use it or provide a link to some literature? The explanation I found on the Esri website was a little confusing for a novice like me (Image classification using the ArcGIS Spatial Analyst extension—ArcGIS Help | ArcGIS Desktop).