OT: But sensibly so, for once...

[Bokkie]

A few days ago I saw a surveyor from the UK Ordnance Survey walking around with an orange backpack containing what must be classified as a professional GPS system and he had a handheld digitiser that I guess he was using to enter data, descriptions etc.

How accurate are the very top-end professional GPS systems and do they work equally well in the open spaces as they do in urban areas?

I mean, he can only see the same sats that I see (if I only had an iQue).

And no! He was not carrying a frying pan, and neither did he have a Gilsson connected to his dongle!

[alexcue]

well i'm no expert on those things, but it is my understanding that the "professional" ones made by Trimble (one of the big boys) if i'm not mistaken are extremely accurate. How accuarate is extremely, well i couldn't say with any certainty, but it's much better than our "commercial" stuff.

I've seen some of those saturday morning archeology shows where they've gotten coordinates from a handheld model and tried to use those same ones on the type with the pot sized dome on them and they were way off. As in about 100 meters or so. The discussion by these Dr's revolved around why it was so far off, and they surmised it was the accuracy differential.


edit: just did a little check at their website, Trimble's that is. They have some that are rated at "sub-meter" accuracy. They use all kinds of add-on stuff (differential, base stations...), that's kinda way over my head.

[Bokkie]

Thanks AlexCue,

this sounds like we need the services of...



The Swedish Contingent...


Gentlemen?

[apersson850]

It depends to some extent for how long you want to wait. With additional mathematics applied (postprocessing), accuracy can be improved down to a few centimeters, or even better.
Usually, when these professionals need high accuracy, they'll have their units sitting in one place for quite a long time, doing measurements. You can also take for granted that they use these satellite constellation calculation tools, so that they know at which hour it will be most favorable to be out there.

[ezjeeper]

Just checked with my coworkers. We use one of those big industrial type monsters to do site surveys for aircraft military stuff. Anyway, it seems that our mil-spec units actually lock on to more sattelites, but the accuracy is down to only about 3 meters. That is what my IQue does on a good day. As a matter-of-fact, we usually take an eTrex or something like that along to double check our military units.

[stephanpls]

Since about 8 years there is the Real Time Kinematic On The Fly concept RTK-OTF). It uses a static base station the position of which is very precisely known. With your mobile unit you can go up to say 25-30 kms from the base station and get good results. I'm out of this stuff for years now so don't ask me the details, and probably the technology has evoluated.

Anyway the positioning precision you could get at that time was about 1 cm. I was on a vessel once and saw that the slight heave movement of the ship was expressed precisely in
the unit's alphanumeric screen: the elevation number of the ship was going up and down centimeterwise. This system (or a successor of it) is widely used now in land surveying. The GPS reception problem between high rise buildings is not solved of course with RTK-OTF; there they still use total stations (that measure electronically angles and distances) for precise mapping.

[NS1007]

on the terminal where I work we will place 3 windmills in due course. We recently decided to replace one of them to another spot. Therefor I had to measure the distance between the new and old location. The old location was measure with professional equipment. Without knowing the coordinates I took waypoints on the old and the new position as best as I could. In a clear open sky receiving as many sattelites as possible.

I had the old position checked for the waypoint results that I got today with the iQ: surprise! only 8cm off!!

NS

[Madmapper]

For anyone interested…

Having some experience in DGPS mapping & surveying we have used various systems for precise measurements. A couple of years ago we had a contract in a Southern African country which involved surveying a large hill to collect precise height measurements over an area of 2km x 3.5km on a 10m x 20m grid. They needed to measure the exact mass of the hill for a planned underground oil storage bunker. The hill was in the middle of nowhere.

The process steps were:
1/ Walk the entire perimeter of the site with a RTK DGPS backpack recording the boundary and then create a grid of the entire area with mapping software on laptop to spit out the (10m x 20m) grid point positions to be surveyed.

2/ Create/build three control points on site (a control point is peg set in concrete and surveyed in to an accuracy of <1cm) at the highest points of the hill in a rough triangle.

3/ Survey in the control points. This required setting up base stations on the nearest three trig beacons (max distance from control point <30km to collect sat data for 6-8 hrs) and using this signal to survey in the control points.

4/ Once the control points precise position and height were known we then setup our base stations on these control points and walked the grid. Navigating to each point and taking the measurements with RTK DGPS systems and the antenna mounted on top of a measured staff (measurement of height of the antenna from the base of the dome to the tip of the staff in millimetres) and using radio repeaters to prevent loss of base station signal in the ravines (out of line of sight).

5/ The height of the antenna (staff) was then subtracted from the “z” readings (height measurements) to give accurate measurements for “x,y and z” (lat,long and height) readings. Approximately fifty comparison spot tests were done with a total station between random points and maximum error on “x & y” readings of 5.5cm and on “z” readings of 13cm were achieved.

6/ All readings were then thrown into a software package to give an accurate 3D rendering of the hill.

Side Note: Guards (young local boys) had to be posted to sit at the base stations all day and prevent animals and birds from bumping into or tripping over the tripods, cables and antennas. Each base station was powered by 2 x 12v car batteries (one for the base station and one for the radio repeater) which had to be carried (along with base stations themselves, tripods, radio repeaters and a variety of antennas & cables) to the top of the hill each morning and back down each evening (no vehicle could get anywhere close to the top of the hill). Temperatures from 9am onwards were in the low to mid 40’s (Celsius) and the hill was covered in thorn scrub. Apart from the three guards at the base stations and my wife (who was along for the ride and to carry the water), I was alone to do the surveying. It took me 5 weeks during December and January, 14 hrs a day (every day including Xmas and New Years day) to complete the contract.

My conclusion? A lot of walking up and down a bloody hill carrying heavy equipment. Damned hot. NEVER AGAIN!!!

Much prefered the cushy contracts of surveying manholes or electrical distribution boxes etc in towns and cities (with a conveniently located café or shop on every corner) with regular DGPS backpack systems (powered by a 12v bike battery) to sub 5m accuracies. Much like Bokkie saw the other day.

I left this field of work almost two years ago (soon after the above mentioned contract in fact) but I believe the new HP (High Precision) systems are now accurate to 10cm (without using base stations).