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stu k1100lt

How to set up units to work with surveys

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I have a Garmin 60CSx and want to be able to set up my unit to play "nicely" with surveyor's surveys.


I often work with survey's and have a need to locate hubs and property corners that the surveyor sets.


These surveys usually work with Northings and Eastings.


Here is an example:


Surveyor sets a mag nail in road and calls it Northing = 10,000.00 feet

Easting = 10,000.00 feet


A hub is then set at Northing = 8702.86 feet

Easting = 10943.28 feet


How do I set up my Garmin to find the hub? Do I somehow use the "User UTM Grid"?


Can I set up 10,000 and 10,000 on the mag nail and then walk to my hub coordinance? How?


Thanks for your help!!

Edited by stu k1100lt

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Good questions. It's a situation I encounter frequently (being in the real estate profession). I have approached it several different ways.


One method to take a GPS reading on a known point, and mark it as a GPS waypoint. Then sent your unit to navigate to that waypoint and walk away until the GPS unit indicates you are the proper distance and bearing from the known point. This assumes you have the surveyor's "calls" printed on the plat. (You will have to convert the bearing into True Degrees. Ask in the Forum, if you've never done this. It's simple.)


Another option is to convert the Eastings and Northings to Latitude and Longitude. The conversion program is on the NGS site. Go to: http://www.ngs.noaa.gov/TOOLS/ and click on State Plane Coordinates. This is a lot more trouble, so I use only for cases where:


*The point with the SPC is also the point I wish to find; or


*The point with the SPC is too far away (I once had a 'tie' which was eight miles distant.); or


*The point with the SPC is inaccessible (locked gate, in a swamp, etc.); or


*I don't know where on the Planet the parcel is located (rare, but it does happen now and then); or


*I can't find any other starting point. (Also rare. Generally, I can find at least one corner pin or a PK nail in the center of the road.)


If you're someone who wants to follow a pointer, there are two was to predict the coordinates of the other points, once you have the Latitude and Longitude of a known point. After making a waypoint at the known point, you can use the PROJECT feature of your GPS unit to create the next point. Use the surveyor's calls for the bearing and distance.


Or, if you have a computer, you can use the FORWARD program in the NGS toolkit to create coordinates, which then can be entered into your GPS unit. As before, you will use the surveyor's calls, with the additional step of converting the distance from Feet to Meters. (The FORWARD and SPC programs can be used on-line, and/or can be downloaded to your laptop for "in the field" use.)


The first time you use one of the NGS programs, it will be as mentally challenging as doing your income tax. [Grin.] But by the third time, you'll be able to breeze through it.




(1) One downside for using Method One is that when you are more than 520 feet from a point, the unit will shift from Feet to Decimal Miles. For that reason, predicting the Coordinates is the way to go for those lot lines which are 1,200 feet long!


(2) Your County GIS site might have a feature which displays the lot lines as a plat, or even superimposed on an aerial photo. It can help you get a feel for where corners are, relative to nearby features.


(3)Some GIS sites allow you to place the cursor on a property corner and read the location in Lat/Long or in SPC. Zoom in a few steps to make certain you are exactly on the desired point. The accuracy will be poor--around 40-60 feet, but it gives you a starting point.



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I successfully set up my Garmin 76S to display SPC meters, or after a fashion, feet. The 60 series can probably do it as well.


I left the unit on NAD83 and selected the SETUP/LOCATION/FORMAT as "User UTM Grid". Then a submenu item Setup User Grid let me set parameters. It took some fussing to find the offsets and scale factor to fit a couple known points (construction stakes with SPC numbers) because I didn't know where to find the official values.


If you are working with an arbitrary origin for your project, you could take a waypoint, read it out in User Grid, and subtract the readings from the previously set False Easting and False Northing to move the origin to the new project.


You can set the scale factor (usually 0.999...) to 3.28... to get feet UNLESS that causes the false Easting and Northing to exceed the range of the display, which on mine is 9,999,999 units. For my area, I have to pick a value to keep the false Northing in range and live with the display being 3,000,000 feet off, an easy thing to get used to. Perhaps choice of a longitude nearer the project would solve that problem for Easting. Even if the longitude isn't the right one for your projection (or if your projection is really Lambert instead of TM) that is probably close enough for work on a local project.


Edit: I fiddled with it some more and optimized the parameters for a 4 by 5 mile rectangle around where I live. I found that changing the longitude of origin slightly let me adjust the ratio of easting to northing. I think this is an effect of the Lambert versus Transverse Mercator. Then I set the scale factor to come out with the right difference between points on opposite sides of the region. Then I set the easting and northing to match in the middle of the region. Some waypoints in this area then read out within 2 or 3 feet of known SPC values, and the corners opposite where I optimized are off about 10 feet.


It would be wise to convert the origin and another distant known point using one of the utilities, set waypoints in Lat-Lon, change to User UTM Grid, and check the displayed values for those waypoints to see if you have the scale and false values right.


Let us know how it goes.

Edited by Bill93

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You can set the scale factor (usually 0.999...) to 3.28... to get feet ----

I know that this is picky, but I believe that the official definition of a meter is 39.37 inches which yields 3.280833 feet. ML

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I can give you the exact parameters to set up a Garmin for a SPC Mercator Zone in either meters or feet or Survey feet.


I think others have such constants posted on the net also, but I worked out my own many years ago.


But it is a radically different proposition to try to shoe horn a Lambert Zone SPC into the user defined system since it is basically setting up a mercator projection. Perhaps it can be done but I am skeptical that it will be very accurate over any significant distance. I fiddled with the idea an afternoon or two and didn't make much headway. It may be possible, but I forget the obstacles that I ran into.


After that the problem posed has other issues. The survey data in question if it has local coordinates will likely not be on either an SPC or geodetic basis of bearing. If you assume that is MAY be close to a few degrees and are willing to deal with that then projecting coordinates in one of those standard systems might work.


Programs like OziExplorer will also allow you to project a new waypoint


Another entire approach is to use some kind of basic COGO to compute the survey coordinates and then apply a coordinate transformation to it based on 2 known points in common between the description and the points on the ground in UTM.


We have many programs that would do this kind of thing, but it is not simple to explain all the possibilities in this limited time and space.


- jlw

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See the "User Defined Grid" section of the following link for step-by-step instructions. http://www.gpsinformation.org/dale/measure.htm


The problem is, as Bill93 pointed out, using a scale factor of 3.2808399 to display the user coordinates in feet causes an overflow condition in the Northing for any location North of about 27.5 degrees latitude as the Garmin 60csx only displays at most 7 digits for user grid Northing. So with this scale factor, North of 27.5 degrees latitude, the Garmin 60csx will just display blanks for the User Grid Northing and Easting. You really have to use a scale factor of 1 (i.e. work in meters in the Garmin), and convert the survey coordinates from feet to meters unless you live in South Texas or South Florida.


Another potential problem interpreting the surveyor's local survey grid (this is what the original poster was describing as opposed to a established State Plane grid), is the basis of bearing for the grid. If the surveyor's grid was relative to geodetic North then no problem. But, if the surveyor was using something else such as an assumed North, then the grid could be rotated significantly relative to geodetic North, in which case the User Grid coordinate values show on the Garmin wouldn't coincide with the local survey grid.


Unlike local survey grids, survey coordinates based on State Plane projections are more readily dealt with using consumer GPS receivers, particularly Magellan receivers, which can deal with both Transverse Mercator or Lambert Conformal Conic State Plane Coordinates. Garmin receivers can only be setup to accurately display State Plane Coordinates for those States that use a Transverse Mercator projection.

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Ok, I missed the local azimuth problem entirely. I assumed they were working SPC with an arbitrary N and E offset to keep the numbers small. I'm doing much the same thing in one of my hobby projects.


The 3.28... was supposed to indicate an approximate value. You don't necessarily wind up with the theoretical meters to feet conversion (3.2808333 for survey feet, 3.2808399 for international feet) because the map scale factor differs from 1.0000 and you are entering the product of the two.


I believe that my approach to "shoehorning" a Lambert into TM is close enough for a limited region of a few miles. My 4x5 mile rectangle is off about 10 feet in the NW and SE corners opposite where I optimized for the SW and NE. Garmin really should catch up on this issue of providing a User Lambert projection.


Edit: Here's someone's writeup on how to do it in Magellan. I don't have that brand so haven't checked it out.


Edit: A very readable tutorial on coordinate systems and datums is available from Michigan State Univ.

Edited by Bill93

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Thanks for all your thoughts. A solution seems a bit more difficult than I had hoped.

I do have a GPS calculator on my GPS that allows me to convert UTM to Lat/Long and find Distance and headings to waypoints.


I'm not needing exact location of the surveyor's hub. I know the accuracy of my Garmin won't "go there". I just want to get close enough to find it in tall weeds. I have a theodolite and laptop that I use to find property the occasional illusive property lines. I just wanted to make sure that I was using my Garmin to its fullest potential.


I will try some of you suggestions and see how it works out.

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If you were to use GPSTrackmaker all you have to do to convert from one to the other is under >tools>options>coordinates is to change it to what you want.

If you want to do azimuths and distance you can do that as well.

I have used it now for about 6 years and use nothing else ....well I do play with other programs but this is the best tool I have yet to find.


Talk about GPS potential........

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