GPS - County Hunting - County Lines
Map Accuracy Standards
Where am I?


On the trip home from the 3M County Hunting Mini Convention in Murfreesbuor, TN in October 2000, I was using a Garmin GPS 12 to let me know how far I I had to go before arriving at the next county. I had placed a route in the GPS which was simply the location of each county line I was going to cross. This was as an aid so that I knew how far it was to the next county. Unlike many county hunters I do not stop on the county line and transmitt from two counties at the same time. The other I had the GPS doing was recording my postiton every few minutes of the trip. This was I was doing just for the fun of it.

I was quite surprised when I arrived at the county line of Wyoming and McDowell, WV on Rt 52 and the GPS told me I was still several hundered feet from the line. That county line runs along the mountain ridge. The sign for the county line seemed to be placed a bit prior to the top of the mountian so I drove on South a bit till I was sure I was past the ridge of the mountian and out of Wyoming County before I began to transmit from McDowell County.

Under the rules of county hunting the sign is considered to be the real line. And under those rules I could park next to the sign and claim to be transmitting from both counties. That is not a bad rule as something must determine the line. The solution is not to say don't transmit from the line and hand out two counties at once. I could park a foot one side of the sign and give out the first county, then move to a foot the other side of the sign and give out the second county. If the sign is not in the correct spot I really was the same county both times. So something must determine the line, and the sign is as good as anyother option. Never the less a misplaced sign is always of interest to me.

When I arrived home I did a further examination on the postion of the sign. The GPS had been tracking my postion so it had recorded several positon fixes as I sat in one spot and tranmitted from McDowell County. These were all within about 20 feet of each other. So I took the average positon and plotted it on the 7.5 minute maps that the U.S. Geological Survey publishes. These are likely the most accurate maps around. At least it will be hard to get a map witch is better and is easliy available. I was surprised when I noted that postion fix that the GPS had given me were not only severl hundred feet form the county line on the map, they were also part way down the mountian. Clearly something was wrong. You can see how far off the GPS spot was on the map at the right. The yellow dot on the map is the positon given by the GPS.

That examination led me to the issue of just how accurate are the map. It turns out they are not as good as I thought. The U.S. Geological Survey actually has "Map Accuracy Standards." And these are published on the web for all to see. The basic standard is that on the 7.5 minute map the postion should be accurate within 40 feet of "truth." This is not quite as good as a GPS can give but a lot better than what I was seeing. But hold on, the real standards are also determined by how they are applied. In this case the U.S. Geological Survey takes seveal points on each map and verifies their accurracy. However the weakness in the testing system is in how the points are choosen. What they take are "well-defined points." These could be the corner of a large building, or the intersection of two roads, or it could be a survey benchmarker. So a single road winding seveal miles thought the woods really has no standards applied to it. The same is true for most county lines. Unless there is a benchmarker on the county line there are no standards. This does not mean that they are inaccurate. What it does mean is that without any quality control on the process there could be "significant" error. In the case of the county line I am seeing what look like errors of 300-500 feet. To most people these kinds of errors are not a problem. To others they are.

Is this situation unique? I have not done a major search however there are other cases equally bad. One interesting case is on Rt. 30 in VA at the county line of Caroline and King William. Here there are two well placed signs, one each side of the road. The pavement has as definite break at the location of the signs. It is as if each county did not want to pave the road as it entered the other county. However the signs are not where the topographic maps places the county line. Rather sign are seveal hundered feet West of where the map claims they sould be. In the map on the left signs are at the point marked by the yellow diamond. As I made two trips to this county line I got smart on the second trip and also got a GPS spot of a small road to a farm house a short distane into King William County. This is the point marked with the red dot. The Topo map does of very good job of placing this road. So as far as the county line which is right - the map or the sign? I do not know.

Least you think all the problems are with the maps I offer one more example. This is one of my favorites as the Virginia Department of Transportation have told me that they delibertly have misidentifed the county line. They did this to avoid confusion on the part of of the public. But it sure confused me the first time I saw it. The county line I am thinking of is where Rt 460 from Richmond to Suffolk, VA crosses from Sussex County into Surry County and then into Southampton County. The road is in Surry County for only about 700 feet. For this reason the Virginia Department of Transportation decided to only identify the change from Sussex to Southamption County. Thus the signs along the road identify a county line that does not exist. This is all done to reduce confusion. The map on the right demonstrates the situation. As it turns out the sign, the yellow diamond, is where the county line signs are placed. They are directly over where the creek crosses under the road. For the county hunters, don't even think or running this line as you would have to park right in the roadway. That is just as well as all admit the marked county line does not exist. I had expected to find that the sign was at least placed in Surry County, but alas it is in Sussex County. What I did find which is nice is that there is a convienent place to pull over on the side of Rt 460 that the maps and the GPS places in Surry County. That point is marked with the red dot on the map. Now I don't have to drive down one of the back roads to get into Surry County. In this case though it is clear that the signs are wrong, so the maps provide the better information this time.

None of this is to say that the underlying Topograhic maps are not of high quality. What it tell us is that county line markings on the map can be in disagreement with the signs placed by the local department of transportation. As the last example illustrate there are compromises in the placement of these signs. They are not intended from the most part to be precise indicators of the loaction of the county line. But also the standards for the Topographic maps as such that they are not readily applied to indefinite items on the map such as a county line. Thus errors of a few hundred feet on such items can be expected. I plan on doing some further explorations in the future. There are many signs that are just where the map claims they should be. There are also many cases where the county line runs down a road and there is only one sign for the county line.

The maps on this page were produced with the TOPO! map progam. In this case I used the Mid-Atlantic USA version of the program to produce the maps. This program is produced by the National Geographic Society. They have a very nice Extended User License that grants legal permission to use copyrighted reproductions from TOPO! In return, they only ask that the corresponding TOPO! overlay files be posted on the National Geographic MapXchange website. I plan to keep posted at that website a file with all of the county lines I have check posted. I can also send a copy of the TOPO! file with the county lines I have checked for use by those who have the TOPO! program.

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