Confusion about ArcGIS Projections
Projections can be confusing in ArcGIS. Specifically, the difference between the “Define Projection” and “Project” tools in ArcMap have completely different functions.
Because if your shapefile or feature class isn’t where it’s supposed to be, you likely need to run the “define projection” tool or the “project” tool.
So which process do you need to run?
Here is a quick guide highlighting the differences between the ArcMap “project” tool and the ArcMap “define projection” tools.
A map projection is a way to portray a curved surface of the Earth on a flat surface.
The ArcGIS Project tool changes the projected coordinate system of your data to another coordinate system. For example, you have a dataset that is currently in latitude and longitude. If you want to transform this data into Universal Transverse Mercator, the “Project” tool in ArcGIS should be used in this case.
Changing the projection by right-clicking an ArcMap will not give the same results as the “Project” tool. You need to run the “Project” tool to change the shapefile. Next time you add the shapefile to ArcMap, it will be projected accordingly.
- The Project tool changes the projected coordinate system in the shapefile, geodatabase, raster, etc.
Define Projection Tool
“Define Projection” in ArcGIS is what you use when the data has no defined coordinate system. The ArcGIS error message would look like: “Unknown Spatial Reference”.
When you run the “Define Projection” tool, the tool does not change the projection. It only changes the metadata describing the current projection of the dataset.
FYI: The metadata that this tool creates is a .prj, .tfw, .aux within your existing dataset.
- Use “Define Projection” when the “Unknown Spatial Reference” warning appears in ArcMap.
- The “Define Projection” tool doesn’t change the physical projection of the data set.
But the thing is:
You have to know what projection the data is in to run this tool.
All ArcGIS projections Tools Are Not Created Equal
“Define projection” changes the metadata for the protection that is associated with the file. The ArcGIS Project tool changes the physical coordinate system in the spatial file.
It’s important to know when to use each tool in an ArcMap session. Although ArcGIS projections can be confusing at times, we hope this guide gives some clarity to projections in ArcGIS.
Now, give it a try yourself.
And give this post some love by sharing it with your friends.
- When you use the “Define Projection” tool, it only changes the metadata of the dataset. It only assigns a new projection in the metadata without physically changing the map projection.
- The “project” tool actually changes the physical map projection for your data set. So after you run this tool, it will use the assigned map projection when you add it to your project.