Positional Accuracy Part 1
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”?
Categories: Data,accuracy,GIS,Mapco,Maps,GPS
Tags: Data accuracy GIS Mapco Maps GPS
I think the real problem is metadata. Few bother to read it, even fewer bother to create it.
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I agree, the way metadata is provided or not provided is certainly one of the problems. When I create GIS data layers I like to put the metadata directly into the database and attach it to each feature rather than have it floating in a separate txt file somewhere.
Unfortunately unless the metadata is as simple as an "estimated accuracy" field the general public isn't going to know how to interpret it.
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There are many problems with metadata, one of them being how the metadata is (are) presented.
Google began showing parcels in Google Maps back in October 2009. There is no metadata for the parcels, but this doesn't stop the general public from embracing the *implied* accuracy of the data -- it's perfect because it comes from Google!
I have spoken to many intelligent and educated non-mapping people on the subject. The near-universal assumption is that the parcels in Google Maps represent property lines.
Of course, we mapping professionals know that the accuracy of the Google Maps parcels is undeclared. As such, they cannot be construed to represent anything, really, much less "property lines." Google failed to make this clear for the general user.
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I can understand professional licensing for public liabilities and maybe union issues but how does that ultimately affect the quality of the data you create. Is there a minimum accuracy standard in the profession or a certain level of equipment you are required to use to be a licensed surveyor. Im not trying to bash the profession, im just trying to understand what the public assurance is since you raised it.
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Let me see if I can describe it this way. If someone from the general public wants legal representation they are going to seek a licensed lawyer. I'm sure there are many folks who have worked at law firms for many years that know the law as well as someone who has passed the bar exam but the person that the general public is going to seek is the licensed person. Why, because the general public has the assurance and understanding that the person who has a license to practice law will be able to deliver the services they require. They also know who will be legally responsible for delivering that service. If services aren't delivered according to the standards of the profession then the licensed person is the one who is liable.
Unfortunately surveyors have not done a good job on educating the public on what our areas of expertise are. I think most people understand that if they need their property line marked they will need the services of a licensed land surveyor. But in general if someone needs elevations measured, coordinates established, or they need anything accurately mapped people should seek the services of a licensed surveyor. Why, because the general public will have the assurance and understanding that the person who has a license to practice land surveying will be able to correctly deliver the services they require. They also know who will be legally responsible for delivering that service.
Unfortunately many surveyors are not familiar with GIS technology, but all licensed surveyors should be experts at field data collection, field measurements, measurements analysis, map production, and coordinate system differences. All skills that would be very applicable to the initial setup and maintenance of a GIS. Surveyors are also trained in quality control procedures, something I often see lacking in the GIS community. When looking at GIS data on a computer screen a surveyor will instinctively ask "where did this data come from, who created it?" "how was it created and how accurate is it?" "what datum and projection was it created in?".
Hope this answers your question. Let me know if it doesn't.
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Accuracy in surveying with GPS- As a civil servant I am always comparing new surveys againsed 20 year old surveys.Are GPS surveys more accurate? Do new surveys confirm by GPS existing benchmarks and control points or do you start your survey from the street corner benchmark that may have been moved when the sweet old lady planted an ornamental?
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I can't say that a new "GPS" survey is more accurate than a 20yr old survey because so much depends on how the survey was done and for what purpose. Someone can use GPS improperly today and someone 20yrs ago may have done a great job with the older equipment used at that time. Stay tuned for my next post on GPS accuracy issues, there are many.
Also, the word "survey" is often used to cover a lot of things. From your description it sounds like you are talking about a survey showing property boundaries. If that is the case then I can tell you that I rarely use GPS alone to perform certified boundary surveys. Usually the only projects that make sense to use GPS for boundary work are very large tracts in open areas, a few hundred acres and up. It is much more common in my area to use GPS to position a boundary survey in the state plane coordinate system to satisfy a local GIS regulation after the legal survey work is done.
So the smaller boundary survey is still done much like it was 20yrs ago, with an optical survey instrument. Then if the customer wants to pay the extra $$$ we can set a high accuracy GPS baseline on or near the project so the boundary survey can be referenced to the state plane coordinate system. Then we would setup the digital drawing file coordinates and orientation using the GPS coordinate information and label property corners on the map with information to match.
Please understand that boundary surveys are first and foremost a legal investigation that must follow the rules of evidence, including discovery of markers moved by the sweet old lady at the street corner. Legal documents such as deeds or maps on file must be analyzed, etc etc. After that is accomplished then measurements are made to locate the evidence uncovered along with the improvements on the property and anything else that is covered under the contract with the client. GPS is one tool that can be used to make measurements but it has it's strengths and weaknesses like everything else. On smaller projects or ones with tree canopy GPS has limited use in boundary work because the accuracy of GPS is limited in those areas.
Hope this helps.
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