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[teletiger]

Is it possible to measure land accurately with a GPS system? Something over 20 rai for example.
Regards.

[Artamus]

Depends on a number of things, and specifically what you define as "accurate".
Assuming you have good satelite coverage, you will typically get an "estimated potential error" (as given on the display of a decent GPS) of 2-4 meters if you are lucky. You can reduce that (fudge it) somewhat by talking repeated measurements at the same point and averaging them (some better GPS models will do this automatically if you set them that way). So effectively all points of your plotted data will be a couple of meters out one way or the other.

A further complication might be the shape of the land. If it's square or rectangular you will have an easier calculation to define the area. Many lands though are not so nice in shape and you may need to split it up and work it out in parts. Of course the larger the area the more accurate the calculations would be, as the estimated errors are a smaller percentage of the actual measured distances.

As an example, if you had 20 Rai (32,000 sq.m.) and it was square it would measure about 179 m x 179 m.
If all your measurements were spot on you'd get an answer of 32,000, but if all were 3 meters off you would get an answer of 33,124 which is an error of about 3%. How accurate would you want it?

[teletiger]

Within 2% would do.   20 rai would be an absolute minimum, so presumably on 40 rai it would be down to 1.5%?
Is there no software that will give a figure after a certain number of plots? Breaking up the area into shapes and calculating manually seems to defeat the point. Might as well do it by tape and get 100% accuracy.
Regards.

[Artamus]

Artamus, on 2009-12-11 10:39:33, said:

Depends on a number of things, and specifically what you define as "accurate".
Assuming you have good satelite coverage, you will typically get an "estimated potential error" (as given on the display of a decent GPS) of 2-4 meters if you are lucky. You can reduce that (fudge it) somewhat by talking repeated measurements at the same point and averaging them (some better GPS models will do this automatically if you set them that way). So effectively all points of your plotted data will be a couple of meters out one way or the other.

A further complication might be the shape of the land. If it's square or rectangular you will have an easier calculation to define the area. Many lands though are not so nice in shape and you may need to split it up and work it out in parts. Of course the larger the area the more accurate the calculations would be, as the estimated errors are a smaller percentage of the actual measured distances.

As an example, if you had 20 Rai (32,000 sq.m.) and it was square it would measure about 179 m x 179 m.
If all your measurements were spot on you'd get an answer of 32,000, but if all were 3 meters off you would get an answer of 33,124 which is an error of about 3%. How accurate would you want it?

Even with a tape measure the calculations will depend on the shape of the land. With any shape more complex than a few sides the calulations get complicated. If the land office do it you will note they place the theodelite in the middle of the land and by measuring the distances to each "corner" and the angles between them they have software to get an accurate measure. Not sure how available this software is, but some smart person could probably do it in Excel with a few functions (it's basically the sum of a number of triangles). It's not as easy to measure the area of complex shapes as it would appear on the surface!

[teletiger]

What I was hoping for was to be able to walk around a border line with a GPS phone, putting waypoints in at every turn and getting the result at the end of a very pleasant walk.
Please excuse my questions if they appear stupid. I am a tec dork.
Would the following software need it's own hardware, laptop etc?

http://www.wildsoft....surveyarea3.htm

Regards.

[Gary A]

Some of the better Garmin hand held units will do a great job. Take your first set of waypoints clockwise and the second set counterclockwise. If the calculation doesn't repeat very closely, do it again. I think you'll be quite surprised how closely it repeats.

ADDED - The GPS does the calculation automatically. You don't need any extra program. If you are going to buy a GPS for this, make sure it is capable of the automatic calculations. You can easily find this information by Googling GPS reviews. It's a good excuse to buy a very handy little device.

Edited by Gary A, 2009-12-11 16:02:05.

[Artamus]

teletiger, on 2009-12-11 12:42:54, said:

What I was hoping for was to be able to walk around a border line with a GPS phone, putting waypoints in at every turn and getting the result at the end of a very pleasant walk.
Please excuse my questions if they appear stupid. I am a tec dork.
Would the following software need it's own hardware, laptop etc?

http://www.wildsoft....surveyarea3.htm

Regards.

This software will only work on a handheld (Palm Pilot etc.). A search on the Internet does not reveal any suitable software for a laptop or desktop surprisingly.

This site
http://www.ja-gps.co...rement-gps.aspx
mentions that this functionality is available on some newer Garmin GPSs, but I have a new higher-end Garmin 60Sx and it does not do this. You might be back to the paper and pencil unless someone else has any bright ideas.

[Gary A]

If you have a GPSMAP 60Cx, it is indeed capable of area measurement.

Features:
Automatic routing (turn by turn routing on roads): yes
Electronic compass: no
Touchscreen: no
Barometric altimeter: no
Camera: no
Geocaching-friendly: yes
Custom maps compatible: no
Outdoor GPS games: yes
Hunt/fish calendar: yes
Sun and moon information: yes
Tide tables: no
Area calculation: yes
Custom POIs (ability to add additional points of interest): yes
Unit-to-unit transfer (shares data wirelessly with similar units): no
Picture viewer: no

ADDED - I had an eTrex Legend and it did a great job. Not all of the eTrex line have that feature.

Edited by Gary A, 2009-12-11 16:42:37.

[Artamus]

Gary A, on 2009-12-11 13:33:24, said:

If you have a GPSMAP 60Cx, it is indeed capable of area measurement.

Features:
Automatic routing (turn by turn routing on roads): yes
Electronic compass: no
Touchscreen: no
Barometric altimeter: no
Camera: no
Geocaching-friendly: yes
Custom maps compatible: no
Outdoor GPS games: yes
Hunt/fish calendar: yes
Sun and moon information: yes
Tide tables: no
Area calculation: yes
Custom POIs (ability to add additional points of interest): yes
Unit-to-unit transfer (shares data wirelessly with similar units): no
Picture viewer: no

ADDED - I had an eTrex Legend and it did a great job. Not all of the eTrex line have that feature.

Hmm, thanks for that. I've been through all the menus on my unit and all through the user manual and see no mention of Area Measurement. I suspect you are right and that it can do this, but I have been unable to find out how. Any suggestions?

[Gary A]

If I remember correctly it is in the Tracks section of the menu.

[Artamus]

Gary A, on 2009-12-11 14:08:32, said:

If I remember correctly it is in the Tracks section of the menu.

Quite right! Thanks for that - very well hidden. It's on the menu button within the tracks screen. Well worth checking for the OP if his/her unit has that function.

[teletiger]

Whilst a new garmin wasn't on the horizon, a new mobile was. Could I kill 2 birds with one stone? Probably. But is the effort worth it. Looking at a Nokia n95.....200+ GPS apps. Erm.......maybe I'll just go with the Garmin.
I'd like to thank you guys for your imput.
Regards.

[Gary A]

The Garmin GPSMAP 60 series are VERY good units and work for vehicles as well as any of the full size units. The cheapest of the series doesn't have enough internal memory and has a monochrome display. I wouldn't buy that one. The CS and CSX models have memory card capability and other than the small screen are great for traveling as well as hiking.

[Artamus]

Gary A, on 2009-12-11 18:09:36, said:

The Garmin GPSMAP 60 series are VERY good units and work for vehicles as well as any of the full size units. The cheapest of the series doesn't have enough internal memory and has a monochrome display. I wouldn't buy that one. The CS and CSX models have memory card capability and other than the small screen are great for traveling as well as hiking.

Yes, I agree. I primarilly use my 60CSx unit in the field, but it's more than adequate for in-car navigation (remember you will probably want/need to buy the holder for it to stick in to the front windscreen of the car for in-car navigation.)

[indothai]

Best accuracy with WAAS enabled is +/- 1.5 meters for consumer products.  But WAAS is dependant on country availability and device capability.

Have you tried Google Earth?.. The maps are about 3 years old.

[teletiger]

indothai, on 2009-12-12 11:15:38, said:

Best accuracy with WAAS enabled is +/- 1.5 meters for consumer products.  But WAAS is dependant on country availability and device capability.

Have you tried Google Earth?.. The maps are about 3 years old.

Apparently only Google earth pro has a measuring capability. I didn't even know there was a pro version.
Thanks for that.
Regards.

[Digitalbanana]

Assuming this is Thailand, WAAS isn't available. So the text book accuracy of a GPS navigation device is 10-15m, due the ionspheric component of the error.
This can only be corrected by using a professional survey type GPS equipment that has a baseline corrected result based. The o/p might just as well pace it out, or use Google Earth if not wanting a professional cadestral surveyor do it.

[LivinLOS]

teletiger, on 2009-12-11 15:42:54, said:

What I was hoping for was to be able to walk around a border line with a GPS phone,

teletiger, on 2009-12-11 19:07:40, said:

Whilst a new garmin wasn't on the horizon, a new mobile was. Could I kill 2 birds with one stone? Probably. But is the effort worth it. Looking at a Nokia n95.....200+ GPS apps. Erm.......maybe I'll just go with the Garmin.
I'd like to thank you guys for your imput.
Regards.

A phone will not be as accurate as a dedicated unit IMO.. Plus the tracking and area calcs are all far easier with a proper machine.