So how do you define accuracy? In the GIS world there are a few different types of accuracy to consider. When some people hear the word “accuracy” they think about the bullseye on a dart board and how close someone can stick the dart near that bullseye. Since spring training just started others may think about a baseball pitcher and how close he can get the ball to the catcher’s mitt or strike zone. As a GPS/GIS person I usually define accuracy as how close the GIS feature is to its true location on the planet, with separate accuracy values for horizontal and vertical location. But accuracy can be defined a few ways in a GIS.

First a few questions; what is the true location of a feature and how can someone find the true location of a feature to tell how accurate their data is? (To be answered later) The second question; does the true location really matter? I have found over the many years I have been working with GIS data that people often don’t want “accurate” data, they want data that looks good. By looking good I mean that the data agrees with the base mapping that is currently being used. So even if the base map is of poor quality as long as the other data overlays nicely everything else doesn’t matter.

That leads me to my multiple definitions of accuracy. In the GIS world we have what I often refer to as “absolute accuracy”, which I use to describe how close a feature is to its true location, as defined by the geodetic datum and adjustment being used. And then we have what I describe as “relative accuracy”, which I use to describe how close to the true distance one feature is relative to another shown on the same map. In this part of the country most land surveys are still done with relative accuracy as the only consideration and I find in the GIS world that digitizing is still very popular because of the relative accuracy visual benefits.

In the early stages of GIS development the difference between relative and absolute accuracy were a big issue. People were trying to use poor quality base maps that sometimes had no real-world coordinates at all. Since then base map quality has improved tremendously. Good thing because as more people wanted to use GPS to add data to their GIS absolute accuracy became very important. Even still I find that the base map and relative accuracy to the base map is the most important concern when displaying data to people that don’t understand accuracy issues.

In my area of the U.S most people are now using orthophotos as their base map. But if I go out and locate the position of a well defined feature using survey-grade GPS techniques and I establish the “absolute accuracy” of that feature to +/-1cm but that location doesn’t overlay onto the base map nicely I find that people want to believe the base map before the feature location. It seems hard for people to understand the idea that everything we look at on a map has an error associated with it and that it's ok for a +/-1cm point to be "off" 80cm when the base map itself is +/-1m. Some features may have more error or lower accuracy than others, even the base map itself.

It seems that relative accuracy to a base map is still most important when viewing the data but absolute accuracy becomes more important when the GIS matures and people want to start uploading the GIS data back onto ever more sophisticated GPS equipment and portable computers for maintenance and/or feature recovery, especially is the feature is buried or difficult to find. The bottom line, use the most accurate base map available, if possible include estimated accuracy values in all databases, make sure all datum references are properly identified, and if you need to “adjust” feature locations to make the data overlay onto a base map better please make a copy of that layer before editing if the original layer has a higher absolute accuracy than the base map.         

Next Post: Where do GIS and GPS positional errors come from?

I’m sure many of you have heard this gripe before but one of my biggest concerns as a surveyor working in the world of GIS is the lack of understanding about positional accuracy. This issue has caused many serious problems and will continue to do so as long as people use GIS data without a real grasp on how data was collected, to what quality the coordinates were produced, and what the proper uses of the data are. Too often I have seen or heard of people in positions of authority using GIS data without regard for these issues.

In some ways I can’t blame the local official who is given a new computer with some fancy data display (GIS) software or access to a new web-site that shows all the relevant layers they need to work with. Instinctively they click on the buttons, display the maps and data they have heard so much about, and go about their job of enforcing whatever it is they are in charge of. But then comes the day when they see a violation of some type showing up on their computer screen and then go about ruining somebody’s day, or year, with the information that GIS software produced for them. Meanwhile it turns out that the data they used to base their decision on is off by 50 feet, or about 15 meters for our international brethren, and the apparent violation doesn’t exist.  

Now I have always been told by those in the legal profession that ignorance is not a valid defense, but in my opinion much of the blame for situations like this fall to those that implement geographic information systems, authorize user access, and procure data. People in these positions must be aware of the GIS limitations and implications if this information is not handled properly. Unfortunately too often the people in charge of making these decisions are not qualified and do not have the experience or training necessary. It is not an exaggeration to say that people’s lives could be physically or financially in danger because of it.

Ultimately this is why societies have instituted professional licensing, to protect the health and safety of the general public. As a surveyor I am licensed so that the public has an assurance and confidence that I have met the standard requirements for my profession. For each GIS data collection project I am responsible for I must determine acceptable tolerances and uses for the data. Unfortunately many GIS’s are populated with data without these considerations. Instead data is downloaded from the cheapest data provider or collected with the lowest cost equipment and/or the lowest cost labor without consideration of the consequences.

It is my hope that each person reading this will think of positional accuracies and acceptable uses before making important decisions with any geographic information. Looking at aerial photos and street view scenes online to find a new vacation spot is cool but it is not cool when someone overlays a parcel layer with a wetland layer or a flood zone layer and finds you in violation of an ordinance or insurance policy. Unfortunately it is too easy for the untrained public to look at this information and think it is gospel. Everyone’s first thought should be, “Is this information I am looking at accurate?”.

Next post: What is “accuracy”?