Plat Coordinates - GPS Coordinates?

Discussion in 'Mapping & Navigation' started by Gert, Apr 5, 2007.

  1. Gert

    Gert Made in Denmark

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    This may be (probably is) a very stupid question, but I know nothing about map coordinate systems etc.:

    Can you / how do you use a GPS to locate a corner of a lot?

    I have a plat (with multiple lots on it) that has each lots corners numbered, and a table that specifies coordinates in the form:

    S 19 43 05 E 181.86

    and if so - how do you enter them into the GPS (that expects a different format) ??
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  2. Tramontana

    Tramontana .

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    Getting to "the point"...
    Are you certain they are giving you corner coordinates, or possibly (and more typical of a survey) they are giving you lengths for property lines, and the angle of bearing for each?

    There should be a "known" point of origin, likely a staked corner that has been surveyed and located on the ground.
    Then you measure a length, along the bearing they give, and sort of connect the dots, following one dimension after the last.

    From the data you provide (S 19 43 05 E 181.86) it would seem to indicate that you have a property line that is 181.86 feet long, and is measured from a point, that is south 19 degrees, 43 minutes and 05 seconds from east.

    Does this make sense?
    #2
  3. Dakar Dude

    Dakar Dude DesertDwellingAdventurer

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    The data provided on a typical plat map or survey would not USUALLY include coordinates. To obtain this, you would need the surveyors point data. And, the unfortunate reality is that even if you had the point data, the accuracy of GPS units available to the mass market are not as accurate as survey equipment. I am intimately familiar with the survey profession, and they are looking at accuracy to within thousandths of an inch...not a few meters or feet.

    Your best bet to locate the corner, is to find a known survey monument in the plat and take a rag tape and start walking...although it sure sounds better to just whip out the Garmin don't it?

    Sorry...
    #3
  4. 9Dave

    9Dave Bazinga!

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    What you have is "bearing" and "distance". These are not directly related to anything you could measure on your consumer oriented GPS.

    Lots of surveyors today are using ultraprecise GPS reception to be able to pinpoint positions on the earth's surface. They use GPS methods and precision not available in normal consumer oriented GPS units. They generate GPS coordinates for the points in their surveys. But not everyone uses this method.

    In basic terms, in the "old" days, you started off at a known point, like a point in the centerline of a road marked by a railroad spike someone had driven there, or even better, a US Geological Survey benchmark that had been found in close proximity, and then used another known point as a reference, and measured the angular difference of a straight line drawn to the next point, and the distance, until you got to the starting point of the property you were measuring. Then from there, you turned the angles and measured distances to each point in the survey. Then you used the reference data of that first line and could say your lines were a bearing of North ___ East, or South ____ West, or whatever based on that, and provide a distance to the next point.

    For the next person coming along, if you found two points of the original survey, you could recreate the whole survey.

    So - based on what you have, there is really no way to recreate your survey without some complex grid mathematics.

    Dave
    #4
  5. RL Lemke

    RL Lemke Long timer

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    AutoCAD files can be exported to a track file using software from MN. I have it somewhere, and have used it to locate the boundaries fo large rural properties. Works well, but requires some computer skill.
    #5
  6. Tramontana

    Tramontana .

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    Getting to "the point"...
    I'm a daily cad user (aka Cad Monkey) and would be interested in learning more about this conversion software.

    Thanks,
    #6
  7. DirtRiderwjc2000

    DirtRiderwjc2000 Been here awhile

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    As metioned earlier your plat map has no useful info on it for your GPS. I have used USGS quad map and a handheld GPS to help locate a section corner (not property corner) the GPS gets me within 20 feet or so.

    btw, I am a registered land surveyor.
    #7
  8. Grreatdog

    Grreatdog Long timer

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    If you are willing to put the time and effort into learning what the information on your plat means, where it comes from and a bit about mapping planes you can use your GPS and plat to find approximate property corner locations. We did it all the time with a pair of cheap Magellan receivers before we got into into using RTK/GPS a few years back. I set the receivers up to work in Maryland and Virginia coordinate systems by making custom mapping zones in the receivers. I would compute approximate coordinates and my survey crews would use them to navigate through the woods on large tracts or to find survey control markers in the middle of nowhere.

    But you can just as easily use UTM (Universal Transverse Mercator) coordinates because almost all receivers will read in those. And you definitely want to be working in some kind of mapping plane using x,y coordinates rather than geographic coordinates (lat and long) and having to use spherical geometry. Trust me on this, trigonometry is a way simpler thing to deal with than having to use the radius of the earth as one of your variables. And UTM is universal so it works everywhere. But first you need a primer on what your plat data means.

    S 19^43'05" E - 181.86' means that property line runs in a southeast direction on an angle measured 19 degrees 43 minutes 5 seconds east from due south for a distance of 181.86 feet. That is a "compass" bearing and is the way surveyors show angular direction on survey plats. The surveyor's compass of antiguity had four quadrants of 90 degrees each. Northeast is measured clockwise (east) from north, southeast is measured counterclockwise (east) from south, southwest is measured clockwise (west) from south and northwest bearings are measured counterclockwise (west) from north. So a bearing of N45^E is actually the same angle on the face of the plat as S45^W but running the opposite direction.

    If you look at your plat hopefully all that will make some sense. Just reading it probably does not. Now you need to understand that those compass bearings were once upon a time based on an actual compass measurement. My (admittedly limited) experience in Georgia is that they are now mostly just a way to relate the survey to some deed and plat from antiquity. But they are still usually somewhat close to a magnetic north based direction. Close enough for this type of thing at least.

    You can get the angular correction to UTM by computation from a number of different sources or you can just look at a USGS topo map of your area. As most of you know those are basically free from a bunch of places on the Internet. Apply that correction from magnetic north to UTM grid north to all of the anglular measurements (compass bearings) on your plat. Make sure you add or subtract in the right direction. It is confusing with compass bearings measured from different points on a compass. So sketch it all out. And it is a large correction so you cannot skip this step. Free software from National Geodetic Survey or a program called Corpscon will do it if you can figure out how to use them.

    Then you need to know what UTM zone you are in in order to setup yur receiver. Here is how Georgia is divided up for GIS purposes (UTM zones are actually divided by longitude):

    http://www.gis.state.ga.us/Clearinghouse/FAQ/UTM_Map/utm_map.html

    Now you need to measure the UTM coordinates of one of your property corners. So set your receiver on it and record a UTM x,y coordinate. Let's assume it is at the north end of the line you mentioned. Now you need to compute the coordinate of the property corner at the other end of the property line in order to navigate to it. On a copy of the plat write the UTM x,y values you got at your starting point. Then draw a line due south from that starting point and a line west from the ending point so it intersects and forms a right triangle. Note that your property line now forms the hypotenuse of the right triangle.

    So you have an angle and a hypotenuse which is all that is needed to solve the triangle and compute the length of the x distance from the start to finish and the y distance from start to finish. First convert 19^43'05" to decimal degrees online or with a calculator (19.7181^). The x distance is simple trigonometry using that angle and the length of the property line. In this case (Sin19.7181*181.86) for an x distance of 61.36 feet. Since you are going southEAST the x coordinates are increasing and you add that to the x value at the start. The y distance is (Cos19.7181*191.86) for a y distance of 171.20. Since you are going SOUTHeast the y coordinate is decreasing and you subtract that from the starting y coordinate. Now you have coordinates for that point.

    Next do that for the property line running from the point you just computed and keep going along each leg of the plat. You should end up within a few hundreths of a foot of the original starting coordinate. If you don't then your surveyor goofed your survey (it wasn't me - I'm not licensed in GA) or you screwed up the calculations (probably because you didn't have a clue what my explanation about bearings actually means).

    Now you can plug those coordinates into your receiver as waypoints and take off through the woods. Remember that recreational receivers have an uncetrtainty of 6' to 15' and your computations have a big uncertainty based on not knowing the true correction from plat north to UTM grid north. But if you did everything right you should be in the ball park.

    Obviously this is pretty pointless on small properties because the GPS errors are too large to get you right to the property corner. Which is what the others are saying. But my point is that it is possible to do from the information on about any survey plat provided you can find a starting corner, you have some idea what the bearings are based on and you can do the calculations.

    My field crews did this all the time to get close enough to know where to start looking for corners when they had to navigate through long sections of woods. Fortunately they also have field computers to automatically adjust the CADD coordnates generated in the office. Even better is that now they are using centimeter accuracy GPS equipment that can localize on any coordinate system.

    Are you sorry you asked?

    Caveat: I am also a land surveyor registered in several states and DC - but not GA. I just do some engineering work for the USFS there that usually requires GPS surveying. I love the TSA guys at the Atlanta airport making me remove wet muddy boots for inspection while my GPS backbacks and data collectors went aboard as carryon luggage without a second look. :huh Maybe they are used to seeing 30 pound packs crammed full of batteries, antennas and electronic gizmos being worn onto airplanes by guys in clean boots.
    #8
  9. bernardnaval

    bernardnaval n00b

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    Below are the said parcels of land technical description in which I want to find the exact location of each lot. Can someone share to me a site or a free program wherein if I enter below coordinates, is it can immediately show to me the exact location. And please teach to me how to enter each coordinates. Looking forward on your help please. Thanks a lot...

    TECHNICAL DESCRIPTION
    (PARCEL I)
    T.P. Cor. 1 is N. 54º 41’ 03.4” W., 577.70 meters
    From BTN 3011, PRS ’92 Control Point
    CORNER BEARING DISTANCE
    1 S. 86º 43’ 32” W. 86.75 m.
    2 N. 35º 55’ 16” W. 277.84 m.
    #9
  10. MrMac

    MrMac Been here awhile

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    Gert, perhaps I'm a bit late on this, but.. You live in Dawson County? They have a good GIS department and I'm fairly certain all parcel data is in GIS. Contact someone in Planning and Development who should be able to get you good map coordinates (State Plane or Geographic) for your parcel corners. Of course, once you have the coordinates, you will still need decent survey-grade units to get on the mark assuming you need that level of accuracy..
    #10
  11. Dolly Sod

    Dolly Sod Red Clay Halo

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    If your plat says "rod found" or rod(f) or pipe(f), or something like that at the property corner, go dig up your corners. If it's a recent survey there may be a pin flag or stake set as a witness to the property corner. If it's an old plat the bearing info is pretty useless unless referenced to a specific datum such as NAD 83. And even if it does reference a datum, the calculation of the appropriate declination for that coordinate base and then the calculation for current magnetic declination is daunting.

    Every year magnetic north moves. So a plat done in 1950 with a true magnetic bearing base will have its magnetic north many degrees different to what it is today. Meaning you can not just find a corner and use a compass derived bearing to sight or walk your property line.

    Are these acreage parcels or lots from a subdivision?
    #11
  12. JRinWV

    JRinWV n00b

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  13. Dolly Sod

    Dolly Sod Red Clay Halo

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    Be careful with that. Your calls are likely to get you tens of feet if not hundreds of fee out of position.
    #13
  14. DADODIRT

    DADODIRT Long timer

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    Why did I have to find this thread?
    Grreatdog summed it up as well as any surveyor could hope to.

    A lot of plats have a 'Basis of Bearing' statement. This will show the surveyors measured bearing and distance at the time of the survey on two monuments(lot corners, section corners(not in GA) or whatever they decide to use. This is helpful because you can measure between these same two points and immediately know your angular difference.

    Keep in mind that the legend will show what type of monument(rebar with aluminum cap, LS37903 OR 1 inch Iron Pipe OR 3 1/14 Inch BLM brass cap stamped.....). If the monument you are standing at looks different it probably is the wrong corner and/or monument.

    The local GIS programs that a lot of counties currently use is good for a starting point, but there can be lots of error in these. One street I know of in Durango shows the lot lines going through the houses consistently for the entire block. That wasn't a surveyor error. GIS=Get It Surveyed.

    I'm not a wiz with the hand held gps units, but understand it better than most.

    Feel free to PM me with any survey related questions as I probably won't check back here. I'm a map guy.:deal
    #14
  15. Shooby

    Shooby Anti-Cager

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    +1000 on this!

    Great explanation by Grreatdog!

    As the other Land Surveyor's (I'm one as well) commented and detailed above, there is a big difference between a layman's handheld GPS and the GPS equipment we use which provide survey grade accuracy.
    I understand the typical land owner / realtor / speculator just wants "to get close" but I've seen scenarios like this end up in litigation because a Professional Surveyor was not consulted until after problems were created by DIY GPS users.
    It's not only knowing how to use the equipment but more importantly, having the experience to perform diligent, thorough research, then knowing how to interpret and use survey / deed / title records.

    Consult your local surveyor, even if it's just a quick phone call (which shouldn't cost anything) can very valuable in the long run.
    #15
  16. JRinWV

    JRinWV n00b

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    Sure seems like we're in violent agreement on the key points: as stated in the programs 'frequently asked questions:'

    Q. I found the deed online, have a smart phone with GPS and the survey lines up in Plat Plotter- can I use that instead of getting a new survey for the land I'm buying?
    A. Plat Plotter does not replace a licensed surveyor. A few of the key benefits of engaging a surveyor include:

    • experience in identifying and eliminating errors in old surveys
    • ability to resolve unclear boundaries and right of ways
    • legal standing to have the revised survey part of the deed
    I need to add a bullet about the inaccuracy of consumer GPS equipment versus professional DGPS, and probably mention declination angle drift.

    DADODIRT makes a great point as well, in that the online county tax records that I have seen look great, but are horribly inaccurate.

    As both Grreatdog and Shooby imply, amateurs (like me) need to remember that those five decimal points of mathematical precision are meaningless if you're holding the map upside down. I wrote the program to convert surveyor data into map overlays and GPS coordinates to help find existing corners on 100+ acres, and largely followed the process Grreatdog described, inspired by conversations with a survey I have used several times buying & selling.

    1/4 acre lot? Dispute over whether the rose bushes are across the line? Different question.

    Want to take a dirt bike out on the 1,000+ acre Hunters Ridge Sports Club but want to be careful to avoid private property? That's what this is for- in fact, that is the example in the quick start guide.

    http://platplotter.appspot.com/lib/pp_help.html
    #16
  17. Dolly Sod

    Dolly Sod Red Clay Halo

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    Here's an example for your 1000 acre parcel. Figure it's a box with 6600' sides. You read an old plat from the 40's with metes and bounds rounded to the nearest degree. You miss figure the declination from the magnetic bearing shown on the plat by 2 degrees. The plat is actually based on a previous plat drawn in 1850 so you're another 2 degrees off. 4 degrees of error down one side of your parcel equates to about a 460' error in where you think the property corner should be.

    Like I said, be careful.
    #17
  18. JRinWV

    JRinWV n00b

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    Declination was one of my first surprises, much along the lines you described. If I remember my lessons from Artillery school, 1 degree at a thousand feet gives about 18 feet of error. The Topos have the angles and annual drift, but who knows when the original survey was laid down. Not sure what the pros use, but that's why I wrote this to be interactive on Google Maps: pin a known point, and look at whether fence lines or roads line up. Add + or - a few degrees and hit recalc.

    Of course, the next problem is road beds and stream beds move over time- we're taking those late 1800 surveys. What I often have to do is run an adjacent property as well- if there is a more recent survey with corners that are distinct on a photo, that can tie in the older Plat.

    The other major issue is there are some bad surveys that have been transferred as is but never reviewed. That Sportsman's club property claims to be 1425.5 acres based on an 1889 survey. I get 1035.3. That's the red flag that makes a survey necessary- and it needs a professional's expertise and legal standing to resolve the discrepancies.
    #18