Have you ever had data that draws thousands of kilometres away from where you thought it should be drawing? Are you unsure about what the difference is between the Define Projection tool and the Project tool? Do you find geographic coordinate systems and projections confusing? If any of these statements apply to you then this blog post is for you.
Working with geographic coordinate systems and projections is not always the most glamorous part of GIS. However, it’s important to understand how geographic coordinate systems and projections can impact your data.
For this blog post, I thought we’d explore the differences between the Define Projection tool and the Project tool in ArcGIS for Desktop by listing some do’s and don’ts for each tool.
One key thing to remember about this tool is that it does not translate, modify or adjust coordinate values. The coordinates remain the same; the only thing that changes is the name of the coordinate system associated with the dataset and possibly the units of the coordinates. Let’s look at an example of the consequences of using the Define Projection tool improperly:
Action: Using the Define Projection tool to change the coordinate system definition of a NAD 83 (Decimal Degrees) dataset that is defined properly to NAD 83 UTM Zone 17N (Meters).
Result: The coordinate values remain the same but the unit changes from Decimal Degrees to Meters causing the data to draw in an incorrect location.
Define Projection Do’s and Don’ts’s
- You’ve received a dataset that has an undefined coordinate system and you wish to assign it the proper coordinate system.
- You have a dataset that is assigned an incorrect coordinate system, which is causing the data to display in an incorrect location, and you wish to assign it the proper coordinate system.
- Use this tool if you wish to reproject your dataset to a new coordinate system.
- Do not guess or assume the correct coordinate system of an undefined \ incorrectly defined dataset. If you are unsure, it is always best to check metadata or the author of the dataset.
While the define projection tool does not alter coordinate values, the Project tool does. The Project tool translates coordinate values from one known coordinate system to another. The result of that translation is saved in a new Feature Class. Below is an example of what happens to coordinate values when you use the Project tool.
Action: Using the Projection tool to project a NAD 83 (Decimal Degrees) feature class to a NAD 83 UTM Zone 17N (Meters) feature class.
Result: The decimal degree coordinate values are translated to coordinate values in metres and the resulting feature class displays in the correct location.
Project tool Do’s and Don’ts’s
- Use the Project tool when you wish to convert the coordinates of a feature class to another coordinate system.
- Use the Project tool using a feature class that has an incorrectly defined coordinate system.
Hopefully this clarifies when to use the Define Projection and the Project tool. The important thing to remember is that the Define Projection tool does not change the actual coordinate values while the Project tool does change coordinate values.