In my previous post, “When Scale Bars Lie”, I mentioned the dangerous things that can happen when the map unit is not set correctly. As I explained, it can lead to abnormal values, which are often manifested in scale bars. This week, a friend of mine was using GIS to calculate the areas of some urban parcels, and getting values in the billions. Once again, the map units were to blame. Setting the map units and display units solved his problem instantly.
As I mentioned before, the easiest way to prevent this issue is to be mindful of projections. I’ve been burned by projection problems before, and on one occasion I had no choice but to restart the project from scratch. So now, I am extraordinarily mindful of projections. Before I begin a project, I choose which projection to use (or ask the clients which projection they prefer). Once a projection has been decided upon, it becomes law – every piece of data is reprojected before being inserted into the project. Admittedly, this is overkill, because ArcGIS will throw up an alert and offer to fix major projection conflicts, and minor conflicts usually don’t add up to much. But I’ve decided that it’s better to be safe than sorry – especially when dealing with unprojected datasets. As the old adage goes, measure twice, and cut once.
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