ARTICLE

Spatial Reference Systems

From PostGIS in Action, Third Edition by Regina O. Obe and Leo S. Hsu

Spatial reference systems: What are they?

Spatial reference systems is one of the more abstruse topics in GIS, attributable to the loose way in which people use the term spatial reference system, and SRS isn’t glamourous. If GIS is Disneyland, then SRS is the bookkeeping necessary to keep the theme park operation afloat.

Geoids

From outer space, our good earth appears spherical, often described as a blue marble. To anyone living on its surface, though, nothing could be further from the truth. The slick glossy surface seen from outer space devolves into mountain ranges, canyons, fissures, and ocean trenches with mind boggling depth. The surface of the earth with all its nooks and crannies resembles a slightly charred English muffin much more than a lustrous marble. Even the idea of the earth being spherical isn’t accurate, because the equator bulges out, a trip around the equator is 42.72 km longer than a trip around one of the meridians.

Figure 1. The geoid representing the earth seen from different angles

Ellipsoids

Ancient Greeks were first recorded as having used the ellipsoids to model the earth.

Figure 2. The geoid and the ellipsoid seen together
Table 1. Ellipsoids used over time

Lon/lat, but which ellipsoid?

Even though humanity has used longitude and latitude for centuries, the longitude and latitude coordinates of yesterday are probably not the ones we use today. Mariners in the Renaissance didn’t have the sophisticated electronics we have now, and their guess at the location of the poles was specious at best. One more thing, earth changes shape over time. This is why it’s important to not only pinpoint a location by lon/lat. You can have NAD 27 lon/lat, NAD 80 lon/lat, and WGS 84 lon/lat, and each is subtly different. A pirate during the golden age of pirates probably recorded his buried treasures using lon/lat significant to one-tenth of a degree. He probably didn’t mention ellipsoid. Good luck trying to find his treasure.

Datum

The ellipsoid only models the overall shape of the earth. After picking out an ellipsoid, you need to anchor it to use it for real-world navigation. Every ellipsoid which isn’t a perfect sphere has two poles. This is where the axis arrives at the surface. These ellipsoid poles must be tagged permanently to true points on earth. This is where the datum comes into play. Even if two reference systems use the same ellipsoid, they could still have different anchors, or datum, on earth.

  • NAD 83 (North American Datum 1983, which is often accompanied by the GRS 80 ellipsoid)
  • NAD 27 (North American Datum 1927, which is generally accompanied by the Clarke 1866/NAD 27 ellipsoid)
  • European Datum 1950
  • Australian Geodetic System 1984

Coordinate reference system

Many people confuse coordinate reference systems with spatial reference systems. A coordinate reference system is only one necessary ingredient which goes into the making of an SRS and isn’t the SRS itself. To identify a point on your reference ellipsoid, you need a coordinate system.

Spatial reference system essentials

Let’s summarize what we’ve discussed about spatial reference systems:

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