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Plotting distance and direction, looking for feature to buy

hikingmike 0 Points
edited November -1 in GPS Recommendations
Trying to help my dad buying a GPS. This is more of an engineering thing than anything else. He has a lot of property and it has been surveyed so he has a map and it has distance and direction from each point to point on the property sub dividing into a few plots. He wants to be able to go to the primary survey marks and use the distance and direction to walk the property and save waypoints for certain things. So, do handhelds have a feature plot feature like that "I want to go 98-deg for 1100 feet?" I know they have a built in compass and will tell you how far you have traveled but I want to pre set that info because if you stray from your initial point then re-aim the compass it won't take you the exact same point. If so, what models or brands am i looking for. I have a basic understanding of gps's from the Army, i've just never went out and bought on. If you all can give me an idea i what i'm looking for i can research from there. Thanks!

Comments

  • Boyd 1985 Points
    Garmin handhelds have a feature called Waypoint Projection that lets you enter a distance and heading. I haven't used it for awhile, but recall it being very limited for survey purposes because you can't enter fractional headings.

    Also be aware that all consumer devices hae an accuracy limited to about +/- 5 meters. So if you mark the same point on two different days they could be 10 meters (33 feet) apart. If you do a survey like this the error will be compounded with each point you mark.

    Paying a little more doesn't help with consumer devices, it just gets you more bells and whistles. You need to go up to the $3000-$5000 range to get more accuracy.

    In my state you can download GIS data that shows the boundaries of all parcels. I think this is becoming pretty common - do some google searches for your town/county with key words like GIS, parcel, download, etc.

    This data usually includes a disclaimer that it isn't survey grade, but I suspect it's better than what you can do yourself. Once you know the coordinates of the property corners, you can go there on different days and mark the location to get a pretty accurate point.

    This can be done with just about any gps device. An etrex 10 could do it for about $100, or an etrex 20 for about $150 if you want to load maps on the device.

    You will need software to open the files you download. I use Globalmapper, but it's expensive. However it runs in demo mode for free and that should allow you to open GIS files and manually record the coordinates or take screenshots (you can't save anything with the demo).

    http://www.globalmapperforum.com/download/
  • Over the last few weeks several clients have asked me to recommend a handheld GPS they could use in order to locate their approximate property corners. You can find that review and recommendation at:
    http://www.propertylinemaps.com/p/plm_Garmin_GPS_recommendation.pdf

    Related spreadsheet:
    http://www.propertylinemaps.com/p/plm_Garmin_GPS_recommendation.xls

    Of the Garmin units currently in production I recommend either one of these:
    Garmin eTrex 20. Buttons. $200
    Garmin Oregon 600. Touchscreen. $400 (I recently purchased one of these)

    As Boyd correctly points out, a Garmin GPS can only ‘project’ a waypoint in whole degrees. Here is a link that starts the Gmap4 enhanced Google map viewer (I am the developer) and shows the difference between projecting a point on a bearing of south 0 degrees east for 1/4 mile as compared to projecting on a bearing of south 1 degree east. According to some professional grade map software I have, the difference is 23 feet.
    http://www.mappingsupport.com/p/gmap4.php?t=s&markers=line=on||44.558161,-85.129146^1||44.561782,-85.129146^2||44.558162,-85.129058^3
    (This forum software does not let me make the above link clickable.)

    But wait - there’s more.

    Surprise! Bearings on land surveys are usually based on an assumed north. In surveyor-speak this is called “basis of bearing”. Often this crucial info is not stated on the face of the survey. The difference between true north and assumed north can easily be several degrees. In order to use the bearings on a survey you need to adjust each one for this difference - which usually you don’t know.

    True, you might find GIS shape files somewhere that you can download and use to display parcel lines. Some GIS systems do a good job of getting the parcel lines in the right location. Other GIS systems - well, not so good. If the property lines are complicated at all then you will likely have more problems trying to pull useful coordinates from those systems. Land surveyors are fond of disparaging GIS systems.

    See the review I wrote for some more tips on ‘recovering’ previously surveyed property corners.

    I wish you success in using a GPS to find those surveyed property corners and if you would like some help getting approximate coordinates for the corners then please check out my service at:
    http://www.propertylinemaps.com/

    Joseph
  • Boyd 1985 Points
    Thanks for your post - some good info here. As a moderator though I must mention that you are treading a fine line when you post links to a website that advertises a service since we have advertisers who pay to be listed here.... :)

    While you are correct about the 23 foot error over 1/4 mile, it really isn't that simple. With each point you plot you'll be compounding the error and it could get very significant if you have an irregularly shaped property.

    I don't know about other states, but the GIS data looks pretty good to me here in NJ, and it is free:

    https://njgin.state.nj.us/NJ_NJGINExplorer/IW.jsp?DLayer=Parcels by County/Muni

    Consider the randomly chosen parcel below. I think you would be very hard pressed to accurately plot this with waypoint projection on a consumer GPS, but it is trivial to open the file and see all the coordinates in Globalmapper (a powerful GIS package).
    Land surveyors are fond of disparaging GIS systems.
    What a surprise! :lol: Have you ever asked these same surveyors their opinion of using a GPS and doing it yourself, or of paying for a service such as the one you offer on the web? Just sayin'.... :wink:

    image
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