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Lesson 5 - Geographic Data Introduction

5.4 - What are projections of geographic data and why are they important?

Projections are used to display three dimensional objects on a two dimensional (flat) surfaces, such as your computer screen or a paper map. There are hundreds of different ways to project geographic data, each with its own strengths and weaknesses.

Orthographic projections are used to preserve the curved shape of the earth. This projection preserves direction and spatial relations but shape is distorted.

Orthographic projection:
	« Notice how the earth is drawn so that it looks like a ball, even on the flat screen or page.

The geographic projection converts the latitude-longitude coordinate system into a square grid. This kind of projection is easy to work with if you are looking at a small area and it preserves direction well. On the global scale, it distorts shapes as you get closer to the poles. The squares formed by lines of latitude and longitude, are square near the equator. As you approach the poles, the lines of longitude converge at the top (and bottom) of the globe, so that instead of squares you get more of a triangular shape on the actual globe itself. When you stretch those triangles into squares, as in a geographic projection, the shapes and areas in the polar regions get distorted.

Geographic projection:

It is important to know what projection any geographic data set is in, so that locations do not get distorted. If you are importing GIS data from another source into TEISS, so you can display it on your maps, YOU MUST KNOW WHAT PROJECTION THE DATA IS IN! If you never import any GIS data from another source into TEISS, then you can forget all about projections. TEISS does not apply a projection to the GIS data within it.

End of Lesson 5.4

Lesson 5.3

Lesson 5.5