Which Map Is Correct?

If seeing is believing, then which map is correct, the survey map or the GIS map? 
                                            

GIS JANET says

Among my first responses to the question "Which map is correct" are "Why are they different?" or "Maybe they are both correct." But I usually know why they are different, and sometimes I can even guess how it happened. However, the question is a valid one that I hear quite often. It seems that, no matter where I travel, a surveyor will say, "You know what really bothers me? It's the way GIS represents the surveyor's work. Sometimes it's accurate and sometimes it's not. It's confusing when my client shows me a map that has been created from a GIS and wants to know why my survey, the one they paid me to do, and the "map" don't agree. I don't know what to say."

Well, I do. And the answer lies in the small print found in the GIS disclaimer. The typical county land records GIS website disclaimer states: The information on this GIS Web site is provided and maintained by various agencies, including county departments, municipal governments, state and federal agencies. No guarantee is given as to the accuracy or currency of any of the data. The Web site is designed to serve as a secondary representation of real property found within this jurisdiction, and is compiled from the recorded deeds, plats, and other public records, which are the primary sources for this public information. Users of this Web site are hereby notified that these primary sources should be consulted for verification of the information presented here. The data layers do not take the place of a legal survey or other primary source documentation. The county and its vendors assume no legal responsibility for the information on this Web site.

So, in essence, the GIS information represented on the website is only as good as the data supplied or as good as it can be when combined with already existing data. The GIS professional is trying to get hundreds, if not thousands, of individual pieces of land parcels to "go together" to represent the most accurate picture possible. But because the data we're using comes from multiple sources with varying degrees of accuracy, the most accurate picture possible may not match your land survey findings. This is why, for years, surveyors have called GIS "just a cartoon," a comment that is not appreciated by conscientious GIS professionals who are trying to be as accurate as possible with the data they have to work with.

In reality, this is not a survey versus GIS situation; it's an end user situation. If the citizens accessing and using the data take the time to thoroughly read and understand the disclaimer, misunderstandings can be avoided.

So does that mean that the GIS data represented in a county land records website should be discounted because the property boundaries are not 100 percent accurate? Absolutely not! There is plenty of highly accurate and useful information at your fingertips, and the property boundary is only one of the many GIS data layers that will help you understand the total picture. Just remember to read the fine print and follow the instructions, and your GIS map will help you understand your world - your way.  

SURVEYOR RANDY says

At last, an easy question to answer. The survey map is the correct map. Seriously, except for rare exceptions, if a county GIS site and a survey plat have significant differences for a particular property boundary, the survey plat accuracy should be significantly better than the GIS representation. Any current survey map signed and sealed by a professional land surveyor should, without question, be the correct graphic depiction of a particular parcel of land.

I have no desire to beat up the GIS cadastral mappers. My staff and I use GIS property information successfully all the time. But we rely on the GIS information only to the extent that is reasonable, because we realize that GIS property mappers have some limitations. A couple of their limitations are the data (deed descriptions and survey plats that are public record) that they have available and the time they can allot to solving and piecing together the property jigsaw puzzle.

Unfortunately, some of the inadequate and/or inaccurate data available to the GIS property mappers comes from surveyors' maps and legal descriptions prepared by attorneys. In North Carolina, particularly in the more rural counties, there are numerous properties that do not have recent or accurate surveys. This is true for a variety of reasons. Maybe the property is not very valuable because it is located in a swamp or the bottomland is not suited for agriculture or development. Maybe it contains wetlands that are now protected from being disturbed or mountainous terrain that is inaccessible. In instances where family lands, particularly agricultural lands, have been passed down to the next generation, there seemed to be no need for a new and more accurate survey.

A surveyor performing fieldwork and mapping can focus entirely on the subject property and the surrounding properties. That surveyor has the most complete and current deed and map information, as well as field evidence such as property corners, adjoining roads, existing public easements, etc. If he has negotiated a reasonable fee for his services, he will have the time to review, analyze, and make defendable decisions regarding a property's shape and size, using all the data he has available.

The GIS property mapper faces a different situation. He or she may be charged with mapping an entire county, and it is simply too expensive and too time consuming to do a boundary survey on every parcel of land in a county; hence, the discrepancies between maps and the importance of the disclaimer.

The good news, and in today's world all of us can use any good news we can find, is that the old, inaccurate data is continually being replaced with new, more reliable data. For example, some city and county entities are requiring electronic digital submissions of new subdivisions and other property maps so that this data can be used to update their GIS with more accurate and complete data.

I agree with Janet that there is a lot of useful information in these GIS sites that is available to a wide range of users. The key is the proper use of that information, including being aware of and acknowledging its limitations and using it only for its intended purpose.


About the Authors

  • Janet Jackson, GISP
    Janet Jackson, GISP
    Janet is certified as a GIS professional and is president of INTERSECT, a GIS consulting firm.
  • Randy Rambeau, Sr., PLS
    Randy Rambeau, Sr., PLS
    Randy is a geomatics office manager with McKim & Creed, an engineering, surveying, and planning firm.

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