A Quick Tip on ArcGIS Projections
Let me guess:
You’re here to find out the difference between “Define Projection” and “Project” in ArcMap?
Because your shapefile or feature class isn’t where it’s supposed to be? Surprisingly, it happens more often than we’d like.
What should you do when a shapefile is located in the wrong location? When working with ArcGIS projections, you need to run the define projection tool or the project tool.
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.
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 “Unknown Spatial Reference” warning appears in ArcMap.
- The physical projection will not be changed with the ArcMap Define Projection tool.
But the thing is:
You have to know what projection the data is in to run this tool.
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 to NAD 83 UTM Zone 11. 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 in order to change the shapefile. Next time the shapefile is added to ArcMap, it will be projected accordingly.
- The Project tool changes the projected coordinate system in the shapefile, geodatabase, raster, etc.
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.
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