ArcGIS Projections: “Define Projection” and “Project”

World Map Projection

Summary

  • 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 in your project.

Confusion for ArcGIS Projections

Projections can be confusing in ArcGIS. Specifically, the difference between “Define Projection” and “Project” tool 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.

Project 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 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 in order 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”.

Unknown Spatial Reference
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 “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.

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ArcGIS projection: Bonne world projection

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2 Comments

  1. So if you add your data in ArcMap and get ‘unknown spatial reference’, does it end up in the correct location? You can add another data set and check where your data is located. If it’s in the right location, then you should run ‘define projection’ and there’s nothing else you have to do.

    But let’s say you have another data set with a different coordinate system… you can use the ‘project’ tool to change your data set to the other coordinate system. This may mean you’ll have to do a datum transformation. Esri will project data ‘on the fly’ according to the first shapefile or geodatabases added in that data frame.

    Hope that helps

  2. If my data has no defined coordinate system. The ArcGIS error message would look like: “Unknown Spatial Reference”. In the next step, I use Define Projection Tool to give it a coordinate system (WGS_1984_UTM_Zone_47N). This creates a .prj, .tfw, .aux within my existing dataset.

    My question

    Next step, I run the Project tool in order to change the shapefile (output coordinate system WGS_1984_UTM_Zone_47N).

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