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When your projection is rejected....

November 30, 2016

You've got your data all nice and neat in an excel file.  You go to add it into ArcMap and your points are all perfectly aligned, but they’re on the opposite side of the earth! 

 

What to do?  You my friend have a projection problem.

 

When I was teaching a GIS lab course to college students, projections seemed to be one of the harder things for some individuals to grasp.  The topic is one of the first things covered in any GIS course- without projections, your data is meaningless!  Many folks dismiss recording the projection used when collecting data in the field, which causes problems later when they try to load points.

 

So, what is a projection?  Basically, a projection is a mathematical transformation to take a round object (the Earth) and replicate it in a flat product (your map).  Over time, people have trimmed, warped, stretched and shrunk the world’s geography in many, many ways to try and achieve the perfect representation.  I used to compare projections to different languages.  The same phrase is different in English, Spanish and German, but represents the same idea.  Likewise, you need to know what “language” you’re working in to communicate correctly.  Think of different projections as languages. If something is written in English, you need to be able to convert it to other languages on the fly.  ArcMap does a great job of managing projections and includes tools to convert your data into one “language” so all the assembled parts align correctly.

 

Projection systems must be combined with a coordinate system to map data.  Coordinate systems provide the “grid” on a projection- the values that are necessary to plot data on the projected earth. There are two types of coordinate systems. The first is global or spherical systems such as latitude/ longitude- these are normally called geographic coordinate systems. The second are projected coordinate systems like the Universal Transverse Mercator (UTM) or Albers Equal Area. Geographic coordinate systems are based on a spherical share and normally use degrees.  Projected coordinate systems are based on a plane (or 2D surface) and are typically in units like feet or meters.

 

Clear as mud?  You don’t necessarily have to grasp all the underlying mechanics in projections and coordinate systems.  The main thing you need to remember is WHAT system you’re working with.

 

But what’s the best projection/ coordinate system?  There’s no correct answer, every situation is different.  The most commonly used projections in the conservation field (in my experience) are NAD 83 and WGS 84. When I was working with South Carolina data, the entire state fit nicely into one UTM zone (17).  Then I moved to Texas, which covers three UTM zones- and most of my data was coastwide. When I used data in Texas, I normally converted files to Albers Equal Area Conic.  Check with your agency to see if one projection is set as the organization standard. Some projections can also have issues when calculating area.  Talk to your GIS staff or search online (Google is a great resource!) for the best projection for your area and situation.

 

Back to the problem- you had data from the Texas coast that’s now showing up in India. Not exactly what you wanted.

 

1. Check the projection on your data collection device.  Many GPS units can be set to whatever projection you prefer.  Write it down! If the data came from someone else, do your best to contact them and find out what was used.  If all else fails, try GCS WGS 84 first.  This seems to be a default on many devices.

 

2. There are two tools you can use to correct your projection, depending on what you started with.  If your data has no projection (like you just loaded it from a spreadsheet or it’s missing a projection file) you’ll use the Define Projection tool (Figure 1). You would hit the square with the hand to the right of the Coordinate System box to pick the projection you need. Notice in this example that there is a warning yellow sign beside the dataset name.  This means that this file already has a projection and this is not the correct tool to use! 

 

 

3. If your data already has a projection, you’ll use the Project tool in ArcMap (Figure 2).  In the

figure, you can see the Input Coordinate System box with the current projection.  You’ll click the button with the hand to the right of the Output Coordinate System box to pick your new projection.  You can define which Geographic Transformation you prefer, but normally the program will pick the default.

 

 

4. In either tool you’ll be directed to the same interface (Figure 3)- Spatial Reference Properties box. Here you’ll navigate through the Geographic and Projected Coordinate System folders to find what you need.  Note that you can set favorites- I have several of my “go-to’s” saved at the top! If I go Geographic>North America I can scroll down and find NAD 83 (note that this is a geographic projection and the values are in degrees).  Or I can go Projected> UTM> NAD 1983 and find the same projection with a different coordinate system- in this case UTMs.  In Figure 4 I’ve selected NAD 1983 UTM Zone 15N (for an area in the upper Texas coast).  Note that when I click on the projection the Current Coordinate System box is populated with the specifics and I can double check that this is what I need. If I think I’ll be using this particular projection a lot, I can hit the star with a plus sign on the far right to save it to my favorites list.

 

 

5.  And here’s the final shot (Figure 4).  I selected my new projection and it’s populated the tool.  I have a default transformation that’s popped up and I’m ready to hit “OK” and be done!

 

 

Projections can be incredibly frustrating!  It’s so important to pick one projection for a project and make sure all the data you use is in (or is converted to) the same projection.  When you add a data file to ArcMap, the projection of the first file automatically sets the projection for the data frame.  You want all your fields to be in the same projection to ensure correctness and valid data alignment.

 

So mind your projections... or your data will go wandering around the globe!

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