I would like to Survey my Property

Frosty214 · Occasional Visitor Joined: Oct 07, 2007 Posts: 5

Hello all,
I have a question that I hope I can get some direction with. What I would like to do is use a gps to help survey a 20 acre piece of land. I was thinking of placing the gps in a stationary position (for lets say 30mins) and recording the data from it. Taking that data to a spread sheet and average it out to a more accurate result. I am not looking for a mm accuracy but log lat and elevation would be the goal.

My first idea was to buy a BT Data logger and then download the info to my computer. Or can I just buy software and connect my current gps to my laptop and record the data. which is that the best solution?

just wondering
john

Andy_P · Posted: Sun Oct 07, 2007 10:52 am Post subject:

Don't want to spoil your fun, but isn't this already on a printed map somewhere? Laughing

You don't say what GPS you have already, but probably none of the driving navigational aids (TomToms and the like) are likely to be accurate enough for you (around 10-20 metre resolution?) and they will try to snap to nearby roads too.
None of the usual "car" models have the sort of data logging output you need either. Something more specialised by Garmin may do?

If you have a TomTom, you might like to have a look at "off-road navigator". The author, "roussillat" can be found HERE

Frosty214 · Occasional Visitor Joined: Oct 07, 2007 Posts: 5

yup that is true, there are maps out there, but I was hoping for something better then 50 foot contour lines. I currently have a Garmin Etrex Summit with altimeter. I have used it to download way points to the laptop and I think the output sentences included the elevations.

I found a web site that explains data averaging and would like to give it a try. http://users.erols.com/dlwilson/gpsavg.htm Although according to this web site it will take a number of days in order to get better than 1m results on the horizontal plan and I will never do better the a few meters on the vertical. Then again this is not using an altimeter. My problem is, what is the best way to capture this data. Do I use a data logger, or buy software to turn my laptop in to a recording device. I am not that excited about leaving a laptop outside for extended periods of time.

thanks for the advice
john

Andy_P · Posted: Mon Oct 08, 2007 12:32 pm Post subject:

Gone WAY beyond my knowledge now, I'm afraid!
Hope someone can help.... Smile

philpugh · Posted: Mon Oct 08, 2007 1:44 pm Post subject:

Some GPS units will average the position for you - but your plan of taking the tracklog into a spreadsheet and doing some maths on it will also work.

The longer term (1/2 day + ) schemes will require some form of logging (either a specific GPS logger or a PC connected) although a GPS should manage. My GARMIN GPS60 will store 10,000 track points - including altitude - setting the log interval to a suitable figure would allow a long period of averaging time. You would have to supply external pwer though.

As for height - you may not be so lucky. You can get the info from the tracklog and do the maths - but there isn't any guarantee that this is going to be close. The geometery of the GPS sats prohibits very accurate height/altitude measurements to start with. If the Earth got out of the way (electrically) then it could be more accurate Laughing

Obviously if your GPS unit is WASS/EGNOS enabled then you start off with potentially better accuracy.

Surveyors use GPS units with DGPS via radio for accurate lat/long measurements but I don't know if either correct for altitude sufficient for your implied needs.

I live on a farm (it's my wife's family farm) and we got 1:2500 maps printed by the OS as you need accurate locations for points such as discharge permits for waste water into streams etc. These are very accurate. They don't have contours on them though. Check the OS web site for specialised maps - they aren't too expensive. The ordinary 1:25000 maps have 5m contour intervals and it's fairly simple to extrapolate the height for a particular spot - also 1:25000 have field boundaries clearly marked.

Products such as Memory Map / Anquet will 'estimate' the height based on actual spot height readings.

The obvious question is why do you need to do it yourself?
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Phil

Frosty214 · Occasional Visitor Joined: Oct 07, 2007 Posts: 5

Oh yes, the reason for project. Sorry I should have mentioned it before. The main point of this exercise it to help plan for the site it self. This would really not be a problem except for the creek and swamp in the middle. The idea is to make a detail (as much as possible) map of the locations and elevations on the property. I would like to know things like if the beavers come back, how high will the water be and where will it end up (in relation to land coverage). For that part a lone I was hoping for accuracy around a 2 inches or 5 cm. From there it would also be helpful in planning things like areas of fill for things like driveways, where to place septic tanks, drilling a well. Overall, just to help plan out the site.

In doing some more research, it might be a better idea to hire or rent a survey quality GPS with DGPS and make key points on site. From there use a transit (I think) and survey the area the old fashion way. Then plotting it back to AutoCad. I just thought that with todays technology it should have been easier to just place a GPS at a spot and get the information I need. Or am I just missing something?

Thanks for the help
john

Darren · Posted: Mon Oct 08, 2007 2:35 pm Post subject:

I assume you're in the US? If so then a DGPS capable receiver (Garmin have them) would be sufficient to get acceptable accuracy of +/- 1m .
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Darren Griffin

Skippy · Posted: Mon Oct 08, 2007 4:29 pm Post subject:

Frosty214 · Occasional Visitor Joined: Oct 07, 2007 Posts: 5

First of all, thanks for all the advice. I really do appreciate it. I think that my original idea might not be a great solution.

Second question...... I was told that the error you have on any given day will be the same for all waypoint and tracks taken that day. For example, if due to environmental conditions my waypoint was off 3m to the north, all the points taken with the same GPS and around the same time will also be out 3m to the north. Is this true?

My newer idea is to have key points professionally marked, and then use them as a reference to adjust for local errors on any other given day. So if I check a professional point on the property and find it is out 2 m south, I could then adjust all that days waypoints 2m south.....Right?

To answer Darren's question. I live in Thunder Bay, Ontario, Canada. The property I am working with is at NAD 83 UTM 16N 318880 5349660 (SW Corner) by 3199320 5349880 (NE Corner). From what I am told, WAAS is a hot or miss up here, but should get better. Unsure when.

philpugh · Posted: Thu Oct 18, 2007 4:27 pm Post subject:

harryparkes · Occasional Visitor Joined: Sep 17, 2006 Posts: 6

Hi,

Reading this discussion caused me to look at some analysis of GPS output I did some time ago. Orginally, I wanted to see if there was any improvement in accuracy from my Garmin Geko if I turned on WASS/Egnos when the unit was locking on what Garmin call Sat No 33. It is not intended for western Europe but never-the-less the Geko had a differential output.

I did 4 x 24 hour runs collecting NMEA data. Runs 1 and 3 had WASS/EGNOS turned on. Comparing individual readings with an averaged one over 24 hours showed no statistical difference with EGNOS on or off. Curiously, the epe - estimated positional error - contained in the NMEA data was 3 times better with EGNOS on!

As to the accuracy of the averaged readings, all four NGR eastings were within +- 0.4 metre. Northings were +- 1 metre and heights +- 0.5 metre. I suspect northings were the poorest because there was some shielding to the north (satellites to the N and S give most accurate northings). On the other hand I would want to repeat the experiments several times more to eliminate chance.

These are relative values. I had intended to check absolute accuracy by parking my car close enough to an OS passive GPS location to get an absolute position. But leaving the car in some remote location for 24 hours was too much to stomach! Not to mention the long walk home.

Finally, this question of averaging values. Looking at the data it is quite evident that averaging over say, one hour is of little value. A satellite lost or gained can make an abrupt change in reading.

If anyone is interested, I have a short .txt note with a summary and some screen shots showing more detailed analysis.

Harry

Frosty214 · Occasional Visitor Joined: Oct 07, 2007 Posts: 5

Thanks to everyone for their help. I think at this point I might just get a copy of Ozi Explorer and map some points and elevations of interest over a picture of the property.

Thanks again
john

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