## Spatial, Geography and Geometry, oh my! – Part 2: Spatial Data (continued)

Last week, we started the Spatial Analysis series with an overview of the two-dimensional aspects of geometry. Today, we’ll look into *coordinate systems, *which include both two- and three-dimensional mapping.

*coordinate system.*A set of three coordinates (1,2,3) can therefore be used to describe the position of a point from an origin in three-dimensional space. (Remember high-school geometry with X, Y, Z coordinate systems? Me neither.) There are many different types of coordinate systems, but when you use geospatial data in SQL Server, you are most likely using a spatial reference system based on either a

*geographic*or

*projected*coordinate system.

### Geographic Coordinate Systems

In a geographic coordinate system, any position on the earth’s surface can be defined using two coordinates. *Latitude* and *Longitude*, and *Altitude.*

*Latitude*is defined as the angle between the equator and a line drawn from the point to the center of the earth.

*Longitude*is defined as the angle between the prime meridian (Greenwich, England) and a line drawn from the point to the center of the earth. Both coordinates are angles, and as such are referred to in degrees.

*Altitude*is quite simply defined as the distance measured from the point to the center of the earth. With that distance, coupled with latitude and longitude, we’ll have a precise point in space.

### Projected Coordinate Systems

A projected coordinate system, as opposed to a Geographic Coordinate System, describes the position of a point on the earth’s surface as they lie on a flat, two-dimensional plane. Think of the Projected system as defining a point on a map, as opposed to a globe. Since we’re projecting on a flat map, we can go back to using the good ol’ (x, y) coordinate system.

That’ll wrap up the defining spatial information. Next up, we’ll take a look at applying these systems to the earth, and how to use them in SQL Server. Tune in on Tuesday!

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