GIS for biologists troubleshooting: Excercise 6, Clipping raster layer using mask layer in QGIS

[Alex T-F]

Hi guys,

I am working through the exercises in GIS for Biologists. In exercise 6, step 3 using QGIS 2.18.12, I am using Raster > Extraction > Clipper and getting the following error message when running the tool. 

"Warning 1: Ring Self-intersection at or near point 2422237.6632541595 1695593.1264573862
ERROR 1: Cutline polygon is invalid. "

I'm struggling to figure out how to rectify this. I have successfully masked other layers before however they are not working in this project. I suspect that it may be something to do with the user defined CRS? Or perhaps a fault with the file 'NORTH_ATLANTIC_OCEAN.shp'. 

I have retried several steps to tease out a mistake but haven't had any luck. 

If anyone has any suggestions that will be great! I have left a screenshot in case it helps

[GIS in Ecology]

Hi Alex,

This is a known bug when trying to do use the CLIPPER tool with the North Atlantic polygon data layer in QGIS 2.18.2. The exercises were specifically written for QGIS 2.8.3 (which is an earlier version than the one you're using), and this is the recommended version for doing the exercises.

There are two possible solutions here. 

The first is to install QGIS 2.8.3, and use it to complete the exercise. You can do this from the links provided on the GIS For Biologists support page (http://www.GISinEcology.com/GFB.htm).

The second is to continue the exercise without the CLIPPER step. This will give you the correct species richness values for the North Atlantic, but it will incorrect value for the small areas of the North Pacific that are covered by the raster data layer (since distributional information on all North Pacific beaked whale distributions was not included in the species richness calculation).

There is also a third option, where you can convert the North Atlantic polygon data layer into a raster data layer, and then use the RASTER CALCULATOR tool to apply it as a mask, but this is a more complicated work-around. If you'd like to try this option, post back on this thread, and I'll post some specific instructions on how to do this.

All the best,

Colin