qgis basics for journalists
Projections: Making maps accurate
Maps have played a critical role in history for thousands of years. They have been used to start wars, make fortunes and topple governments.
One of the great advances during the Age of Discovery was the refinement of latitude and longitude. Amerigo Vespucci (the cartographer who named the Americas after himself) used the moon and Mars to calculate the most precise measurements of latitude of his time. Even though the maps were fairly inaccurate, they helped the Spanish navigate oceans, circumnavigate the globe and amass tremendous wealth. At the time, maps were considered state secrets.
Governments are still trying to improve the accuracy of maps. Satellites, sonar and even the space shuttle have been used to improve our understanding of geography.
But fortunately, many maps are not considered state secrets and governments are a primary source of map files (e.g. U.S. Census Bureau). The challenge of making public maps is creating a standard that everyone can use. That's where geographic information systems (GIS) come in.
GIS generates accurate maps by converting a 3D globe to a 2D plane. Latitude and longitude create a coordinate system, which gives us the ability to "unwrap" the the earth's surface.
But creating a square map out of a globe requires us to distort the geography. The closer one gets to the poles, the more the surface is distorted. That's why Antarctica looks 7 times larger than the contiguous United States in the example below. In reality, Antarctica is only about 60 percent larger.
The example above is one of two types of coordinate systems. Specifically, it's referred to as a geographic coordinate system or geographic projection. There are many versions of geographic coordinate systems but the most common are NAD 83 (used by the U.S. government) and WGS84 (used by GPS devices and Google maps).
But the distortion inherent in geographic coordinate systems creates maps that misrepresent the shape and size of geographical areas. To adjust for that, cartographers create projected coordinate systems.
Projected coordinate systems often take a portion of the globe and unwrap it in a way that will more accurately represent either the shape of a boundary or the area of a boundary. One of the most popular projections in used North America maps is Albers Equal Area Conic Projection. Here's how it compares to a geographic projection:
The Albers projection is a more accurate representaton of the size of the United States.
Setting your projections
So here's the tricky part. Projections distort maps to create a desired result. Most files you get from sources will have a projection assigned to them and they may have to be converted to match your project. There are hundreds if not thousands of projections and some won't easily convert to others.
I set all my projects to NAD 83 or WGS84 as a default. That's also QGIS's default coordinate system and it seems to convert easily. But you need to know how to change your projection before you publish your project.
Click the Edit Projection button in the bottom-right corner of the status bar.
This opens the Project Properties. Check the Enable 'on the fly' projections box, then you can choose a projection from the library, search for one or access one you have used in the past.
To set your projection, click the Apply button. Then click on the Layers tab to make sure the projection is applied to all your map layers.