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Thread: why I got a GPS and Topo map accuracy

  1. #1
    Senior Member buddy's Avatar
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    why I got a GPS and Topo map accuracy

    I,m an advid bushwhacker/orienteering hobbiest and for many years used map and compass only for backwoods exploration. I was in an area of woods near my home in western MA and found that the map did not seem to corelate with what I was seeing in the woods. I assumed that my navigation was flawed and I was not where I thought I was on the map. The following week I went back to the same area with my map taped to a piece of cardboard and my compass in hand. I constantly referenced the map and navigated with the most precision my abilities would provide and still found that the map was not corelating with what I saw in the woods. There was a gently sloping wooded slope where a wet land should have been and in another area a year round stream was missing. Assuming the map was correct and somehow I must have screwed up again I decided to purchase a GPS to help me improve and check my navigation skills. To my surprise the GPS varified that my navigation was correct and the map was...well... wrong? I have found other areas around western MA where the USGS topo maps have minor errors, some seem to be cases where data was inverted and others seemed to be simply bad quesses by the cartographers, they can't possibly verify every square mile of the mapping area. A little google research revealed that some areas are better mapped than others. Areas around civil engineering projects such as roads, bridges, utility lines, railroads etc. are well surveyed and mapped. Public parks and goverment owned land tend to have accurate maps as well. However, large areas of privately owned lands may not have been accurately surveyed and that fact is reflected in topo maps more often than I realized. Summits, obvious points of interest, waterways and major property boundaries are accurate but some of the topo info in between can be completely fictional. Am I out of my mind or have others found this to be true as well.

  2. #2
    Senior Member TCD's Avatar
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    I'm in the Adirondacks. The maps are usually pretty good. Here are the "exceptions" I've found:

    Because the maps were mostly made from aerial photos, small features may be missed, even if they "should" show up at the level of detail on the map. Small bumps and dropoffs that well exceed the topo interval sometimes do not show up.

    The more recent maps have undergone some post processing and are "computer smoothed." Significant features that might appear on an old field checked, hand drawn map can be missing, especially cliffs. I have stood at the top of 100' tall dead vertical cliffs, where the map does not show any 20' contour lines touching. On the map it's a "steep hill."

    Things can change on the ground. While in our area large scale topographic features seldom change, smaller features can change over time. Streams can disappear due to some activity upstream. Ponds can silt in, or can greatly increase or decrease in size due to beaver activity. Landslides can dam up or reroute streams.

    Man made features can change a lot, and quickly. These are the least reliable landmarks. Roads are rerouted; buildings are removed and others are built; property lines are resurveyed, etc..

    It's all part of the game!

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    Senior Member paul ron's Avatar
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    There have been many times I've questioned the paper map, it wasn't agreeing with where I was but eventually you will hit a hand rail.

    I've found big storms have rerouted streams n changed some of the topography but only on a small scale. When looking at the big picture, you'll find the maps are pretty good.

    I've been carrying a cheap GPS (ETREX basic yellow) to do an occasional reference check between way points n paper map, just to see I'm still on track when the map says differently. The GPS can do what a map can't.. point to my destination from anywhere I am standing.

    .

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    Senior Member DougPaul's Avatar
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    This is an old story... Maps have errors. GPSes have their own errors too, but are often more accurate than available maps. Many have been confused when confronted with the discrepancies...

    USGS topos are based upon aerial photos with surveyed ground-truth points. Anything that cannot be seen in the photos cannot be mapped accurately (eg trails under tree cover.) Also some features are seasonal or changeable (eg some hydrological features) and may be missed or shown in an unusual phase. And, of course, some features have changed since the photos were taken.

    More recently, a terrain mapping radar was flown on the Shuttle (SRTM: Shuttle Radar Topography Mission http://en.wikipedia.org/wiki/Shuttle...graphy_Mission). This produced elevation maps (DEMs = Digital Elevation Maps) of much of the planet, however the resolution is limited (30 meters) and it couldn't identify features. Some have generated topos from the DEMs, but they are often inferior to the USGS topos.

    Some computer maps plot the USGS topo data with underlying DEM data so they can print out the altitudes of tracks and points.

    Doug

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    Senior Member RoySwkr's Avatar
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    Quote Originally Posted by buddy View Post
    Am I out of my mind or have others found this to be true as well.
    The USGS is very careless about details in unsettled areas, there are plenty of spots where the 15' shows 2 bumps and the 7.5' one, and when you get there it's 2

    They also value presentation over accuracy and leave off whole contours if it's hard to draw them all, Lizard Head in CO is an extreme example but The Nose VT is another

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    Senior Member Nessmuk's Avatar
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    Quote Originally Posted by TCD View Post
    Things can change on the ground. While in our area large scale topographic features seldom change, smaller features can change over time. Streams can disappear due to some activity upstream. Ponds can silt in, or can greatly increase or decrease in size due to beaver activity. Landslides can dam up or reroute streams.

    Man made features can change a lot, and quickly. These are the least reliable landmarks. Roads are rerouted; buildings are removed and others are built; property lines are resurveyed, etc..

    It's all part of the game!
    That's the thing. Work from the biggest solid features, those that most likely are not mapped or plotted incorrectly (mountains, hills, deep gorges), then work down to smaller features that you observe in the immediate vicinity. Everything you see is a potential navigation "clue". Right down to the statistical frequency of moss on the side of trees if you take into account everything that would make moss grow one way or the other in that location. Make sense out of all the clues nature presents. Something seems out of place or somehow doesn't make sense with the others? Understand why. Understand that objects, both natural and man made, can change. There could be perfectly reasonable explanations such as the examples TCD suggests, but will not include a mountain being located someplace (or not) unexpected because of earthquakes and volcanoes - just so you can justify a location that you "know" you must be at.

    There are some surprisingly very old USGS topo maps that are the most current available, especially in the Tug Hill region west of the Adirondacks. There are a couple that have a latest revision date as early as 1943. My father grew up in that area and said there were no beavers at the time as they had all been trapped out much earlier. But now the place is overrun with beavers. The relatively flat terrain of this broad plateau, as the source of many creeks heading in all directions, is literally swarming with beavers and their dams. There are ponds both active and inactive everywhere now, none of which are shown on the map. Navigating by map is an exercise in either prior knowledge, or in simply expecting that any lowland could have a dam anywhere.
    "She's all my fancy painted her, she's lovely, she is light. She waltzes on the waves by day and rests with me at night." - Nessmuk, Forest and Stream, July 21, 1880 [of the Wood Drake Canoe built for him by Rushton]

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    Wow! Thanks for this.

  8. #8
    Banned Kevin Rooney's Avatar
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    As others have pointed out, maps are not always accurate. Tim (hikesbikesdoesalotoffishing) recently pointed out in relation to the "Brutus Bushwhack", there's an error in Garmin's MapSource as to the location of the brook in relation to the trail at the the beginning of the bushwhack. I'd never noticed the error until Tim pointed it out because the "mental map" (see the NYT's recent rather silly article on that subject) in my own head knew the BW began immediately after crossing the brook. So, even widely used maps contain errors.

    Having said that, my advice is - trust the maps, as they're nearly always right, and this is especially true if you're lost. But, also trust your own senses and experience, as they're of value also.

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    Senior Member sardog1's Avatar
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    I've told my navigation students in the past that when you start noticing map errors and feel confident in that, you'v learned your lessons well.

    And now for a shameless plug plus a chance to embarrass Nessmuk, all in one fell swoop. If you check out the VFTT Pathfinders social group, you'll find a veritable library of useful resources on navigation. Perhaps foremost among these is Nessmuk's treatise on the subject, which I goaded him into providing a link to. Ignoring its lessons is "Not Without Peril".
    Last edited by sardog1; 02-09-2012 at 06:41 AM.
    sardog1

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    og Drykk og Trste og det heile, som
    er Liv og Helse i ein Hovedsum."

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    The Missing Mountain

    I remember a cartographer who shall remain nameless telling me about expereinces GPSing the existing trails where the trail is on the right of the stream and when the GPS trrack was laid into the USGS data it was shown on the left of the stream. Seems there is only a certain (lower) degree of precision for waterways in the USGS data. So in this case, the map can't show the trail on the wrong side of the stream. Do you manipulate the GPS data to"move" the trail in the right and correct side?.....

    I found a similar situation where the intersection of two streams coming down major drainages in the Pemi occured about 200 feet lower than shown on the USGS....

    Then there is West Tumbledown Mtn in ME.....
    "I've been kicked by the wind, robbed by the sleet, had my head snowed in, and I'm still on my feet, and I'm still,...willin"

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    A somewhat related question

    As a preface, I have no experience using a GPS, and I have a survey map of a piece if property. Can a GPS be used with the coordinates on the map to find
    the boundary points of the property? The property has two survey monuments and one marked pipe that I can use as starting points, but the monuments are not at all the corners and we are hoping to find the side property lines. This is vacant overgrown land with no landmarks, and of course the neighbors have hazy memories of the boundaries.

    Trying to save a few bucks as monuments will cost us $500 each. We don't need need the information to enforce a boundary, just want to get a better idea of where the land ends. Any advise is appreciated.

    Hugh.

  12. #12
    Senior Member RoySwkr's Avatar
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    Quote Originally Posted by HughK View Post
    Can a GPS be used with the coordinates on the map to find
    the boundary points of the property? The property has two survey monuments and one marked pipe that I can use as starting points, but the monuments are not at all the corners and we are hoping to find the side property lines. This is vacant overgrown land with no landmarks, and of course the neighbors have hazy memories of the boundaries.
    A good GPS will allow you to "project a waypoint", that is from a known point you enter the bearing and distance and follow the needle until you're there. Of course this location is only approximate. Repeat with all bearings shown on the map - if you have bearings and distances of all the side lines from the known points you will know the approximate locations of the corners. If you and the neighbors can agree on the locations, set pipes there.

    A professional surveyor can survey the location of the pipes and create a plan that if signed by all neighbors and approved by the planning board can be recorded as the legal boundary regardless of where it previously was.

    I am not a licensed surveyor and some members here are, if they answer believe them :-)

  13. #13
    Senior Member DougPaul's Avatar
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    Note that the nominal accuracy of a consumer GPS is 95% probability of being within 10 meters--not generally good enough for surveying one's property. Over short distances, you can do better with a good compass (the best hiking compasses are accurate to ~2 degrees) and a long tape measure if you have good local reference points (monuments).

    Professional survey grade instruments can be accurate to a few cm or less.

    If you want anything more than an unofficial estimate of your boundaries, I'd suggest that you talk to a licensed surveyor.

    Doug

  14. #14
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    Quote Originally Posted by paul ron View Post
    There have been many times I've questioned the paper map, it wasn't agreeing with where I was but eventually you will hit a hand rail.

    I've found big storms have rerouted streams n changed some of the topography but only on a small scale. When looking at the big picture, you'll find the maps are pretty good.

    I've been carrying a cheap GPS (ETREX basic yellow) to do an occasional reference check between way points n paper map, just to see I'm still on track when the map says differently. The GPS can do what a map can't.. point to my destination from anywhere I am standing.

    .
    I have one to and love my basic yellow fella. I track and track back with it.
    How do you do a reference check between way points n a paper map. I would love to try that. PM me if you like and have time to reply. Thanks!
    Last edited by Maddy; 02-15-2012 at 07:25 AM.

  15. #15
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    A survey on a property line is good insurance for the future when the land may not be vacant or the neighbors change. One thing to settle with the neighbors up front is do they agree with the corners and do the property boundary descriptions match? If either one is the case the price of the survey goes up as the surveyor has to do research to determine what the actual points are. If its a relatively new subdivision done by a surveyor its a quick process. Surveyors are licensed professionals and they have to be able to stake their reputation that the survey they did is accurate so they cant really just cut a line between two unknown points and call it good.

    Most people want points along the property line and possibly blazing. That is usually done with a surveying station (high tech transit). Usually GPS points are used to locate the corners. If the woods are open, the surveyors will run a traverse line along the line zig zagging as needed to avoid trees, they then loop back to the point where they started to ensure ther accuracy and then after some office work, they can go back and mark the property boundary by tunning angles and distances from a known point that they established during the initial traverse. A lot of the field work is cutting brush, sometimes you can work out a deal with the surveyor to do the hacking and slashing instead of paying someone to do it.

    If the line is several hundred or thousands of feet long I expect a GPS will be adequate to keep everyone happy for points along the line. Just turn on the WAAS and let the GPS sit for an extended period to get whatever accuracy it can give you. In most states, a private individual can not legally put permanent markings to mark property boundaries but pink ribbon lasts if its maintained every few years. If both parties agree that they will agree on the ribbon thats usually good enough. Of course when its a timber cut and that prized veneer log is just right over the line, that can be a two or three thousand dollar decision.

    One thing to keep in mind is that especiually with older rural lots that the accuracy of the original survey and subdivision generally wasnt very great and frequently they tie back to even rougher documents. If there is a question, the surveyor has to establish the initial intent of the original survey and how the abuttors have used the land in the past to eventually come up with an informed opinion of where the line should be.

    Years ago, I found an error in a deed for my brother that gained him about 15 acres of land on his property. It was a modern subdivision of an old parcel but somewhere in the process the new surveyor blew it. It was an internal error in the subdivision so they just cut a new line in the woods and called it good.

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