The importance of the correct use of projections in GIS data

21 Mar

Hello guys,

Today I was explaining somebody about the consequences of messing up the geographic coordinate systems and projection when you are working with GIS data.

In reality once you decided which projection you are going to work with, to have all your data with the same projection or to transform data that is in another projection is very simple. However, the problem comes when you mix data with different projections such as, for instance, a DEM with one CS and a shapefile with another. Although software like Arcgis warns you when you are working with data in two different systems, it doesn’t stop you from doing spatial analysis or any other kind of work. To illustrate this problem (that can be quite serious if you get it wrong), I’m going to show you the difference using two point shapefiles with THE SAME DATA but DIFFERENT PROJECTION.

The red dots represent archaeological sites that were recorded in ED_1950_UTM_Zone_30N and the black dots are the same sites but projected as WGS_1984_UTM_Zone_30N

Different projections

Although the difference may not seem large (182.923 m), in fact, when performing spatial analysis this will have its impact in the results. I know that in archaeological cases this might not be an error that cause a major disaster like calculating wrong, for instance, flooding levels of a damn. However, it is highly important that you get it right in order to have correct results.

There is a very useful resource where you can find the main geographic_transformations_complete, parameters and areas of use released by ESRI that can be used with all “project” tools in Arcgis.

Ok, I hope this is useful and see you soon 🙂

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