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Understanding projection and coordinate systems

From: Up and Running with ArcGIS

Video: Understanding projection and coordinate systems

Whenever you make a map, you're abstracting And here I'm going to choose this one This creates a map that all but eliminates the distortion from Washington

Understanding projection and coordinate systems

Whenever you make a map, you're abstracting information about the real world and placing it on to a flat plane such as a computer screen or a piece of paper. Since the planet earth isn't flat, some distortions of the data will inevitably occur during the transformation process. These show up as distortions of area, shape, direction, distance, and scale. Various map projections try to minimize the distortions, but no projection can eliminate all distortions at once. Coordinate systems, on the other hand, are how we locate a point on the Earth. Typically, we think of the latitude and longitude values of a GPS receiver as being the coordinates of a place.

But it's only been in the last few years that GPS coordinates have been so easy to obtain. There are many other ways to locate a point. The state plane coordinate system, for example, was designed to make surveying easier. Measuring the number of feet north and east of a standard location that was used state wide. An in-depth review of the math behind these complex geographic transformations is well beyond the scope of this course. For now, though, it's good just to know that ArcMap has built in support for lots of different projections, and that it can usually transform your data on the fly between the various systems.

Now, this map that I have on the screen now is currently in the WGS84 coordinate system that is used by GPS receivers. We can change the projection by double clicking on the layers reference map here. And visiting the Coordinate System tab of the data frame properties. Now, I'm going to go up to the very top and I'm going to collapse the geographic coordinate system folder and expand the projected coordinate systems to take a look in here. Let's go ahead and scroll down and I'll open up the world folder and within that I'm going to scroll down and I'm going to find the Mercator projection here.

And go ahead and say Apply. Let's go ahead and move the Data Frame properties window out of the way so we can see the updated map. Now, the Mercator projection distorts the surface of the Earth in such a way as to create a map where straight lines represent lines of constant compass bearing. This is great if you're trying to sail a ship in the 1500s, but it does this at the expense of creating highly distorted land masses at higher latitudes. The classic example is to look at the size of Greenland up here, and compare it to the size of Africa here. In the Mercator projection, Greenland looks to be about the same size as Africa, but in reality, Africa is about 14 times bigger than Greenland.

Now, for a long time, the National Geographic Magazine used Robinson projection in order to reduce the distortion in world maps. Let's go ahead and take a look at what that looks like. I'm going to go back to the Data Frame properties. And I'm still in the World folder. I'm going to scroll down and find the Robinson projection here and press Apply again. And the map will update back here. Let's go ahead and move this out of the way again, so we can see the updates. So, this is what the world looks like in the Robinson projection. National Geographic has since shifted to the Winkel tripel projection in order to further reduce distortion. Let's go ahead and take a look at that one.

Let's scroll down. And let's use Winkel tripel. And press Apply, and once again, the map updates again. Now, for larger scale maps, you're better-suited to use a projection that's designed for the specific area that you're interested in. Let's go ahead and close the data frame properties for a moment, and I'm going to go up to my bookmarks and choose the United States bookmark. And you can see that this probably isn't a good way to represent the United States. It has this very slanted appearance here. Let's go ahead and change our projection so that it's one that's suitable for this map. Go ahead and double click on the layers again and this time I'm going to scroll up I'm going to close the world folder and I'm going to scroll up a little further until I find a continental folder here and inside of that I'll go to North America and I'm going to choose the Lambert Conformal Conic and scroll down.

I'll go ahead and press Apply and it's going to warn me that it's going to go through a geographic coordinate system transformation. We'll go ahead and say Yes, we'll accept this change, and the map updates in the background. I'll move this out of the way again. Now you can see that this projection gives the traditional bowed look to the United States. Let's go ahead and take another look at a closer view. I'll close the data frame properties once again and this time I'm going to use my zoom magnifier to zoom into the state of Washington here. Now, you can see that the state of Washington appears slanted and so this probably isn't a projection that's suited for that.

For state maps, there's individual state reference grids that can be utilized. Let's go back into the Layers property here. And the coordinate system. Once again I'm going to scroll up, close out North America and close out continental. And this time I'm going to open up the folder called state plane here. And I'm going to choose the NAD83 US feet coordinate system. And now we can see that we have different coordinate systems that are set up for each state. I'm going to scroll down until I get to the Washington group. And here I'm going to choose this one here, this NAD83 state plane Washington south.

Go ahead and press Apply, go ahead and accept the transformation warning, and we'll say OK. This creates a map that all but eliminates the distortion from Washington state, and orients the map so that it is balanced across the page. Now, if we zoom out to a world view again, we'll see that it's definitely not suited for a world map. So understanding the complex nature of the various projection systems that you'll come across, when exploring geospatial data, is a key component to becoming proficient in ArcGIS. There's lots of different pre-defined systems to choose from. But probably only a couple that apply to your particular part of the world.

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Up and Running with ArcGIS

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