26 May 2015

New Toy: Survey Grade GPS

I have a new toy! Oh joy, a new toy! This isn't actually a new piece of geophysics equipment, but survey equipment. Having previously set out grids using a total station and recorded data on an arbitrary grid, I can now get absolute coordinates, set grids out much quicker and with only a single person. I went for net rover rather than rover plus base as I wanted decent coordinates in the field rather than post processed.

I eventually chose a Javad Triumph-LS. There were several things that attracted me to this particular model. It received all signals from all constellations (so futureproof and accurate), it was cheap (relatively!) and the company had made an effort to deal with interference due to the lightsquared debacle, which is good for me, because part of its job will be sitting on top of my radar antenna. The unit even gives you a relative quantification of interference it is receiving. I put it on top of my radar, turned off, and the value was 4. Turned on, it was 22. Rather than having the receiver on top and data collection half way down the pole, both are combined at the top of a mono-pole. It does seem a bit top heavy and difficult to keep steady, but it will correct itself for pole tilt, which is a nice feature. Control is via a number of hardware buttons and a capacitive touch screen like a smartphone, so the whole thing seems very modern. Getting started with the kit, the support on their forums was excellent. On the downside, I found a few things unintuitive, but most of those can be put down to me being a geophysicist rather than a land surveyor. The receiver has not long been released and the manual still needs a bit of work to deal with the likes of me.

So that's the hardware, which I'm very pleased with. I also needed a network RTK corrections subscription. In the UK, all of the correction services are based off of the base stations run by OSNet from the Ordnance Survey. Of the five different services that use OSNet hardware, two are aimed at farming. While these are cheaper, they only connect you to the single base station, which is fine if you are near one, but not very good if you are not. You can only expect to get 5cm accuracy with these services. The other three services, SmartNet from Leica GeoSystems, VRS Now from Trimble and TopNET live from Topcon do things differently. They create a model of the atmosphere based on a number of base stations and create a Virtual Reference Station based on your current location, giving you a potential accuracy of 1cm. Apparently all three services produce a similar level of accuracy, but they don't give any information on which constellations/signals are corrected or whether they provide any additional base stations over and above those provided by OSNet. Price wise, at the time of beginning to look at all this, their websites showed the Leica and Topcon services charged £1200 ex vat per anum for the limited (40 hour per month) service, while Trimble charged £1500 or £1300 sans SIM card.

Trying to sign up to one of these services turned out to be a tale of woe. I started out contacting Leica. The contact email address and price on the website was wrong (now fixed) and the price is now £1260. After speaking to someone by phone and then emailing, contact went dead and they stopped replying to my emails. Not very good if they don't even want to sell you something. I next tried Topcon. I filled in their web form and got an email saying that my registration was confirmed... then heard nothing for a week. Next I tried Trimble. I filled in their web form, which then demanded that I give them my VAT number. Not even having a company, I don't have one of those, so Trimble fell at the first hurdle. They were too expensive anyway. I went back to Topcon, having noticed another contact email in their confirmation email. Once I had the attention of a human, service was prompt and helpful. There were further problems, like the first SIM card they sent got lost in the post and the second one was the wrong size (mini instead of micro), so I cut that down to size, but I messed it up and cut it slightly wrong.  They sent me a SIM of the correct size and all was well in the end, so Topcon came out on top(con). Connecting to the service, I got my desired 1cm accuracy. According to the receiver, it was receiving correction for GPS + GLONASS. Hopefully, the Ordnance Survey will upgrade their base stations to cover Galileo now that constellation is coming online.

One thing I wanted to test is that the receiver supported the black abomination that is OSTN02, a modification of OSGB36. I surveyed a random point in ETRS89, converted it internally to OSGB36 and did the same conversion with the Ordnance Survey's own coordinate transformation tool. Initially, it seemed it didn't, as the difference was about 2.06m. It turned out that I also had to change the datum used by OSGB36 to the Newlyn datum, which made it produce the same values as the OS site, so all good there. I also wanted to test how accurate Google Earth imagery was locally, so I recorded a bunch of points in ETRS89, converted them to WGS84, as used by Google Earth and converted it to kml. Here is a picture of how that turned out. Looks like the Google Earth imagery is very good indeed!

How do I use this fantastic new toy in the field? With the total station, I used to set up the total station at one end of a baseline along a straight edge of a field, tell it that it was at 500E,500N, point it along the baseline, then tell it that it was facing something like an angle of 270 (west), even though it wasn't. Then I could just go and look for 460E,500N etc. For reestablishing the grid, I used to record two resection points that could be described to a few cm, the downside being that those points could disappear, which did happen in a couple of cases. For placing on maps, I recorded a bunch of points at the edge of the field to match to the edge of the field on maps or aerials. The process is described in more detail here.

With the GPS, I start by collecting the same point I originally would have occupied with the base station, then setting it to stake out to that point and walking 120m away along the baseline. I then do a Multiple Point Localization on those two point, using coordinates of 500E/500N and 380E,500N, which will create a new projection which I can use to walk to further points just as with the total station, only with a single person instead of two. The two points used for the localization are recorded in lieu of resection points, but the new projection is stored on the device, so they are only needed if other people are re-establishing the grid. Since I get absolute coordinates, there is no need to record points around the edge of the field to overlay the geophysics correctly, so everything becomes much faster and more reliable.

Now to get out and use it. I'm going to be doing a big radar survey of Chichester in July, so watch this space for some results from that.

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