GPS for surveying

[Ali Wood]

Apologies if any previous topics relating to this.
I wanted to ask readers if they had any experience of using a GPS watch/device to assist surveying orienteering areas, how accurate it is when pinpointing point features, which product they would recommend, how to transfer data from device to computer, what are the pitfalls etc. etc. I'm not a GPS user up till now.

[Red Adder]

I use a Garmin that is over 5 years old and I would say that they can sometimes help, sometimes not. The best accuracy is about 5-10 m, though at times, in dense woodland, they can "jump" to be 20-30m or even more out. In parkland / school / largely open areas they can help and can even be uncannily accurate at times.

In general though the older models are not really up to the task as primary mapping tool as if followed as gospel would only produce an inconsistent and therefore inaccurate map. The best use I have is an aide memoir of the route you took - after +5 hours in the field it can often help you sort out your ideas and notes in to the correct sequence.

In EA, where admittedly mapping can be relatively simple compared with other parts of the country, the most effective assistance comes from base maps that you personally create from aerial photo sources such as google. In my experience you should be able get the basic layout and all the key features accurate to 2--3 m; far better than wheel and compass. This is done within the comfort of your own study, leaving all the valuable time in the field for interpretation of what is on the ground, rather than exhausting yourself just to produce the basic layout.

[Ali Wood]

Thanks Red Adder for that. Of course I accept the basic starting point must be photos and google whatever, but I see the main advantage is in areas where there ain't that much help from aerial photos (because of tree cover but also when you're mapping a contour feature not obvious from the air like a shallow re-entrants). In this situation I have often wanted desperately to not have to pace out for the fifth time to a track when things don't match up. I could cope with GPS being 10 or 20m out, as long as it gave me a basic idea of where a feature was for starters.

[Freefall]

If anyone can suggest a package or "app" to load onto an Android Phone (which has GPS) please let me know. I use the excellent Viewranger with OS 1:50,000 background maps normally but would to import my own georeferenced files as a background (OCAD/shp/dxf/jpg). Format doesn't matter too much as I use Global Mapper to save into different formats as required.

[Sloop]

Alistair,
I have a Garmin Forerunner which is a few years old now. I use it absolutely all the time when mapping, planning and controlling. It is a mistake to expect it to provide definitive positions all by itself - but it is nonetheless very helpful.
Mine has two settings for data recording - Every-Second and Smart. I tend to use Smart as I get up to about 11 hours of battery life and potential recording that way, versus about 2 hours on every-second, but Smart does tend to give less accuracy.
I scan the base map, and use some old Australian software called O-Track to load in my GPX file, and overlay it on the map. This is a pretty simple tool, and I suspect there's much better, and possibly free, software available now. For controlling and planning, that's good enough to give happiness that features are reasonably well mapped and especially where things lie in relation to each other.
When mapping, as well as ensuring I have a track covering my every move, I keep a log of what I was doing ... eg. 43-47 mins walking the perimeter of patch of brambles, 56mins loop round small depression. ie. when I've decided to map something I make sure I have a GPS record of whereabouts it was. I also pace distances and add those to the log. This, combined with whatever you can remember, seems to work pretty well so long as you do your OCAD as soon as you can after your survey. eg. suppose you've decided to map a series of point features in a triangular area between solid boundaries ... pre-GPS you would probably do pacings and careful bearings from each corner to nearest feature and between each feature. With GPS you can do without the bearings bit, and probably halve the amount of pacing. For contour features, you can try to simply walk along the contour (my Garmin has an altimeter but I doubt its accuracy, and certainly not under tree cover); I also walk the middle-of-ridge lines and bottom-of-reentrant lines.
If you combine all the above with sufficient walking along known features (eg. the path network) you can exercise appropriate discretion when looking at your wiggly line, and produce a decently confident map.
Hope this helps,
John

[Glucosamine]

You get what you pay for - and subscribe to. Basic GPS can vary as accounts above - especially if the signal is obscured - though the correction systems on recent models can help considerably. Smartphone chipsets can have good specs, and are getting better, but rogue readings are probably caused by poor antennae. Subscription services can provide centimetre accuracy in good conditions - farmers use these to keep sprayers on the same tramlines to minimise soil compaction. Using a moving marker on large scale (1:10000) OS mapping on an HTC smart phone for work is generally pretty close - but not always. Routegadget plots tell a tale ...

[dustytoo]

My Garmin 60 is now a bit old. As per other posts it suffers from poor accuracy in heavy woodland, even more so if the woods contain steep-sided declivities like reentrants and gullies. Depending on the availability of known points in line of sight, I find using a laser rangefinder can be a helpful substitute for GPS for pinpointing some features. If neither GPS nor rangefinder are usable, then it's back to doing it the hard way.

[Red Adder]

How useful do you find the laser rangefinder Dusty ? Someone I know who has one was clearly not overly excited with it - finding it hard to guarantee getting a definitive fix when in woodland - sounded like you needed a partner with a reflector (and protective glasses) to stand at the appropriate point.

[Ali Wood]

Seems like everyone has an 'old' Garmin - be interested to hear from anyone who has a more recent model...?

[dustytoo]

The rangefinder is really useful, as long as you work within its limitations. As my pacing is hopelessly erratic it is quicker, easier, and more accurate to use the rangefinder. You need line of sight, obviously, and slopes are a problem, but generally I find it a useful additional tool in the armoury. A reflective surface is needed, especially in gloomy woodland. I have used the services of a willing colleague to hold the reflector (I find a white carrier bag is as good as anything), and have surveyed a small jumbled area of banks, ditches, walls, and gullies in this way. Usually it is just me, and I mostly dangle a carrier bag on a stake. That seems to work, even if you do have to cover the ground twice to move the stake. Next week I will be experimenting with a shiny helium party balloon to see if that will do the trick for fixing features on slopes.

[AndyC]

What he doesn't mention is the use of participants in a "Gay Pride" event to hold the other end of the measure and the reflector while updating the map of a Newcastle Park- he didn't however manage to get any of them to turn up at the event a few days later

[marcusgeoghegan]

Freefall asked: "If anyone can suggest a package or "app" to load onto an Android Phone (which has GPS) please let me know."

Try trekbuddy on Google Play. It's a very nice way of getting a georeferenced bitmap of your O map onto your android phone.

[Nimby]

Google's My Tracks
https://play.google.com/store/apps/details?id=com.google.android.maps.mytracks&hl=en

[Pete_Bog]

Freefall asked: "If anyone can suggest a package or "app" to load onto an Android Phone (which has GPS) please let me know."

OCAD maps can be directly loaded into your android phone using the Momap app. This app is useful for viewing a map as it is continuously zoomable. Whilst it has an editing feature, this is impractical on small screen, might be OK on a tablet?

In the meantime, Open Orienteering Mapper are just about to release (tonight?) their next iteration of the excellent open source OCAD alternative. This version should be capable of being used for real rather than just for testing

[Freefall]

Excellent - will have a look. Just changed my phone to a Motorola Defy Mini - screen seems a bit small but it is waterproof and dustproof and allegedly has a very good battery life. Runs Android so I will investigate apps.