Migrating to GPS-based Coordinate Systems

GPS has made it possible to make highly accurate, low distortion maps covering the whole world. Coupled with the ability to measure location quickly and cheaply (or very precisely for a reasonable cost), many new applications have been made possible. However, the transition can be challenging and span a very long time. I’d like to outline some of these challenges and discuss how software fits into the puzzle.

What Came Before?

Before GPS, locations were primarily determined by placing monuments in the ground, and then measuring the angles, lengths, and sometimes heights between them. The curvature of the earth was accounted for, but only in a way that made sense for a local area. The resulting distortions, originally acceptable for many applications, are a deal-breaker for accurately mapping over wide areas. They also result in coordinates that aren’t directly usable with GPS.

Hurdles

The main hurdle with the transition is the huge number of maps based on whatever pre-GPS system was in use. These represent tremendous value (examples range from maintaining property boundaries to managing natural resources) and can’t be discarded. In some cases, it’s desirable to keep coordinate values roughly the same (or perhaps intentionally different, so old and new coordinates can’t be confused). These existing maps won’t disappear overnight, so the need to transform coordinates both ways (old to new, new to old) remains for a long time.

Heights pose a special opportunity and challenge. While GPS determines location in 3D, it doesn’t capture “height” in the traditional sense; you can’t directly use it to find your elevation above sea level or figure out which way water will flow. There are known solutions (e.g., based on geoid models or grids) but the key point is that there are choices. If an existing height system is in use, it may be desirable to maintain or stay consistent with it.

The Need for Local Control

The transformation between old and new coordinates is necessarily approximate due to measurement error and (typically large) distortion in the old system. So, one might argue that a variety of transformation methods are okay, as long as they stay within the accuracy of the distortion. However, it’s common for a single transformation to be mandated by an appropriate authority. This provides a variety of benefits, including allowing the old system to be redefined in terms of the new, which means it can continue to be used even as the monuments that help define it fall into disrepair.

The demand for single, official transformations, coupled with the great creativity and motivation of the transformation designers, has resulted in a large number of transformations for software to support. Some of these transformations are based on standard algorithms (e.g., Helmert) or file-formats (e.g., NTv2), but others are more exotic and are provided via software libraries or detailed specifications.

Are We There Yet?

Although GPS has been around for some time, some countries are still in the middle of transition. This makes sense given the need for great care, and the huge amount of data to be transformed. Even once this is done, further (albeit smaller) transitions await, as higher-precision applications and approaches are developed (e.g., the transition from NAD83 to CSRS in Canada or to HARN/HPGN in the US).

New Threats

While I’m confident GPS will survive into the future — primarily because of the phenomenal value it provides — it’s not immune from challenges. One example is the Lightsquared debate in the US about a proposed network that might interfere with GPS accuracy.

Conclusion

The transition to GPS-based mapping has brought significant benefits and challenges. For us, the most exciting part is helping transform all that data! How have you been affected by the transition to modern coordinate systems?

PS: In this post, I’ve done my best to avoid technical terms. Earlier, I provided detailed overviews of coordinate systems, datums, and heights; or if you have more time, you might consider the delightful summary from Ordnance Survey’s guide to coordinate systems in the UK.