You may remember those big plastic maps that hung on your elementary school wall - the ones where Greenland was as big as South America. you've been messing around with mapping software and notice it puts New York about 600 miles further than it really is to Los Angeles. What's going on? Was your elementary school actually right? Is your computer telling you that L.A. is further out than you thought? This shows how you can get tripped up with maps. The short version of the problem: The world is round. Maps are flat. When map makers try to convert our sphere onto a flat sheet of paper - or computer screen - something has to give. Maps are always going to get something wrong. The trick is getting them to give you the right answer to your question.
Don't hesitate to befriend a local geographer, perhaps at a university or planning agency, to make sure you are using the right projection.
The things that maps can get right or wrong is the shape of a region, its area, the distance from one place to another, and the direction.
The solution is picking the right projection. Every projection gets something right. And every projection gets something distorted. No projection can do all things perfectly all the time.
The mapping column in the March issue of Uplink showed how different maps use different calculations for the shape of the Earth. This is called datum. A latitude and longitude from one popular datum could be several hundred feet off from the datum used by your hand-held GPS unit. If you didn't know that, you could mistakenly report, say, that the scene of a flood was outside a town's official flood plain. That column outlined how to convert a map's datum.
Projections
Projections are different than datum, though they are often mentioned in the same breath. Datum are used for knowing where any measurement sits on the planet. Projections try to convert our planet into a flat map. Every electronic map has underlying datum. But not every map is projected. And projections - which are essentially a lot of math - are always built on top of some underlying datum.
There are hundreds of projections. And the things they try to get right, as they flatten our planet onto our computer screen, are: area, distance, direction and shape.
The map on your elementary school wall, with Greenland the size of South America, was not doing a good job of representing area. Put a coin on part of Greenland, and it might cover eight times the amount of land in real life, than if you put that same coin on part of South America. The computerized map that calculated New York much further than reality to Los Angeles was distorting distance.
Yet these same maps may do an excellent job of preserving shape and direction. Sailors used these kinds of maps to navigate. They could follow a straight line on their map and stay on course as they curved around the world.
If you've negotiated for a map, and a government agency tells you they keep all their maps in a State Plane coordinate system, that's a cue that the agency's maps are already projected.
For instance, a North Carolina map using a state plane coordinate system uses a projection called Lambert Conformal Conic. It's a projection that works well for elongated states. It does a decent job for measuring distances, and has minimal distortion of shape and area in small settings. But the direction from the northwest corner of the state down to the southeast corner is distorted; it's an arc, curving around the planet, rather than a straight line.
To make matters more confusing, not every State Plane uses the same projection, so you would have trouble edge matching a series of State Plane maps if you don't run conversions.
If you've heard that a map's projection is "Geographic" - like many U.S. Census Bureau geographic files - that means they are not projected. You'll need to project them depending on the analysis you want to do.
In any case, be sure to check out a map's metadata - the detailed file describing any electronic map - to learn about its projection before any conversion.
Converting maps
Don't convert your map if you don't need to. Think about the analysis you need to undertake. The reason is that every conversion involves estimation, and the more you convert a map from its original projection, the more locations could be imprecise.
Sometimes the name of the projection tells you what it does best - "cylindrical equidistant" or "Albers equal area." Direction tends to be best preserved with azimuthal maps. Shape is represented well with conformal maps.
Keep in mind that there is always some amount of estimation involved in using any projection, so it's important to be familiar with the projection you are using.
If you are only analyzing a small area, you may be able to get by using just your local Universal Transverse Mercator zone or State Plane coordinates. Keep in mind that there is always some amount of estimation involved in using any projection, so it's important to be familiar with the projection you are using. Also, to spare you a shock: a given projection will no longer refer to a point in its latitude and longitude, but in its own coordinate system.
Some handy reference books are listed on the margins of this column. Also, don't hesitate to befriend a local geographer, perhaps at a university or planning agency, to make sure you are using the right projection.
Analysis
When it comes to analyzing data, you'll probably be most concerned with preserving distance and area. Distance would be a concern when, say, you want to buffer to determine what's within some miles of a hazardous chemical spill. Area would be an issue when, for instance, you want to calculate the population density within that buffer.
As discussed in the March column, the new version of ArcView comes with something called the Projection Utility. You can run it separately from ArcView. Just click on the icon on your desktop.
Once you're in the utility, the first step is to point and click your way to the shape file you want to convert. Hit the next button. For step two, first check the box that says "Show Advanced Options."
Let's assume the data is not projected, so keep the coordinate system at "Geographic." Click the datum tab and highlight the correct datum, say North American 1927. Hit the next key. For step three, select the projection you want to convert to, like a particular UTM zone.
The selection should automatically fill in the correct datum, but it's still a good idea to check and be sure. Hit next. Step four is simply to navigate to where you want to store your converted files. Now you can hit the next key and then finish. The utility will create a reference file that keeps track of the projection and datum for your new file.
The conversion utility is not perfect. Those who wrote the code for this utility noted it may be off by a meter or more.
Andy Lehren can be reached by e-mail at alehren@nbc.com