What Every GIS Manager Should Know, But Likely Doesn’t

They are always moving, faster in some geographic regions than in others. It seems someone is always coming up with a different method to interpret them. Once you start using a certain method, it’s very difficult to change. Updating them in a large-scale GIS can be a nightmare. What is it?

Of course, I’m talking about coordinates: the G is GIS.

It’s the G in GIS that makes your GPS navigation unit work. It’s the G in GIS that powers Google Earth, Bing Maps, the local parcel database in your community, and the 911 routing for the emergency services in your community.

Generally speaking, the accuracy of G is pretty sloppy in most of these apps, several meters in many cases. But, the general public doesn’t see that because the GPS unit gets Mom to the right soccer field and the GPS unit gets the emergency crews to the right house, at least close enough for common sense to take over, like GPS guiding an airplane close enough to the airport for the pilot to take over and land the aircraft.

This raises an interesting question, and one that I get a lot from people. How accurate is…(insert noun here)? How accurate is GPS? How accurate is Google Earth? How accurate is my community parcel database? How accurate are the USGS (or your federal map agency) topographical maps? How accurate is the digital nautical chart I’m using?

Usually, folks think their system is “pretty accurate.” I’ve heard boaters swear that their consumer GPS unit takes them back to the “exactly” same spot, not within 10 feet, but “exactly.” I’ve seen architects use Google Earth to lay out their master plans, expecting that the construction people can accurately lay out the design from their plans.

The bottom line is that much of the digital geography in the world is not very accurate, largely because it doesn’t need to be. Well, at least it didn’t need to be at the time.

Now, that’s not to say that many GIS parcel databases don’t fit together very tightly. Most of the ones I’ve seen do have a tight fit, but a tight fit doesn’t mean it’s accurate. We should briefly revisit the difference between accuracy and precision. If you want to read more about accuracy  vs. precision, I wrote an article about a year ago on the subject. Otherwise, here’s a couple of graphics to illustrate:

Figure 1. Very precise, but not accurate.

Figure 2. Not very precise, but more accurate than Figure 1.

I use a phrase that one of my Dad’s friends used to tell me. He worked for the U.S. Geological Survey. He would say, “I’d rather be generally right than precisely wrong.” That sort of sums it up. Of course, ideally you’d want the dots in Figure 2 to be tightly centered in the bulls-eye so then the data would be very accurate and very precise, but I think the point is clear.

How do we define what is accurate?

Coordinate accuracy is actually an arbitrary thing with respect to geography. Who is to say how accurate a coordinate pair is? One person may say the coordinates for a point on the ground is x, y while the other may say the same point on the ground is x1, y1. The two pairs of coordinates may differ by decimeter, a meter, tens of meters, or more.

Who is correct?

Essentially, everyone within the region (country, countries, etc.) needs to agree on what the accuracy reference is.

Typically, there is a federal government agency within each country that is tasked with defined a coordinate reference frame. It just so happens in the United States the organization that defines and maintains the National Spatial Reference System (NSRS) is a government agency called the National Geodetic Survey (NGS). Each country roughly has its own version of the NGS.

So, in the United States, when someone asks what the “true” coordinates are for a point on the ground, the answer is the coordinate pair for that point as referenced to the NSRS. There are several ways to determine the coordinates for said point, with one of the more common and efficient methods being an accurate GPS receiver (survey quality) that references another GPS receiver (survey quality) that is already tied into the NSRS. The NGS OPUS service is a neat way to accomplish this because the NGS already provides access to GPS receivers that are tied to the NSRS.

Allow me to use another example that I’m commonly asked about.

Many times, I’m asked by GPS users how accurate their GPS receiver is. Usually, the easiest and quickest way to determine this is to visit one of the tens of thousands of survey marks located throughout the United States (other countries have the equivalent). The NGS publishes coordinates for each one of the survey marks. Now, be forewarned that there are some caveats. The coordinates of some survey marks are not that accurate. You’ll want to use one that has been surveyed using GPS. You can find the closest survey marks by visiting this website, selecting Radial Search, selecting GPS Sites Only, and entering your latitude/longitude. It will display the closest survey marks to the latitude/longitude you entered. You’ll be surprised how many there are and how close they are.

There are some cool, free software tools available that allow you to easily find an NGS survey mark near you. One that I became aware of recently (a la Brent Jones/Esri via Twitter) is an Android app created by Critigen. It’s pretty slick. I didn’t find the app by using the “Market” app on my phone so I just downloaded the FindAControl.apk file from Critigen’s website and tapped on it to install the app.

If you’re really itching to find out how accurate your GPS receiver is, this is a great place to start. Yes, you probably won’t do something right the first time and probably want to do it again (and maybe again), but it’s a valuable learning experience. Actually, to get a good statistical sample, you’d need to collect a lot of data at different times and on different days to get an idea of the accuracy consistency of your GPS receiver. Because, as I’ve heard many times, and maybe you, too, even a broken clock is right twice a day. But that’s a subject for another time 🙂

Thanks, and see you next time.

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