mrewenmacdonald, post: 411135, member: 11707 wrote:
My guess is that there is something up with the scale factors..
So now I have no idea whether my total station is lying to me or my GPS is lying to me. Any Ideas? ways to figure out which it is?
Ok, lets simplify this for the op.
Total stations are affected by:
Air density (temp and pressure)
Prism offset.
Total station offset. (the point of measure, is not made right at the axis, but at the location of the coaxial edm, in the unit, and corrected, to be at the axis.)
None of these are affected by going to scotland.
Gps, however is affected by world travel. You need to carefully select what projection the lat longs are projected to, and that, i think is at the core of your troubles.
If i use a projection in Hawaii, or England, while geographicly being in Arkansas, it's going to never work right.
I think your troubles are in your Gps settings.
Reset the projection, for scotland, then for scale factors, you only have grid to ground, which is probably less than 100 or 200 ppm.
Ok? Another way to visualize it, is to consider that people on the other side of the earth are walking around, with their feet towards us.
N
Nate The Surveyor, post: 411325, member: 291 wrote: Ok, lets simplify this for the op.
Gps, however is affected by world travel. You need to carefully select what projection the lat longs are projected to, and that, i think is at the core of your troubles.
If i use a projection in Hawaii, or England, while geographicly being in Arkansas, it's going to never work right.
I think your troubles are in your Gps settings.
Reset the projection, for scotland, then for scale factors, you only have grid to ground, which is probably less than 100 or 200 ppm.
Ok? Another way to visualize it, is to consider that people on the other side of the earth are walking around, with their feet towards us.
N
GPS provides earth centered - earth fixed coordinates (ECEF). As both John & Paul pointed out, the comparing a simple inverse between GPS derived ECEC coordinates and the corresponding TS derived distance is projection neutral and scale has no impact.
As far as projections go. Mathematically it does not matter were on earth you apply any given projection as long as they are applied correctly to terrestrial measurements. Using the correct projection merely limits the distortion between grid and ground in a given area.
Use the here function and create a local projection and GPS should match your total station, but once you start using state or utm projections you need to be aware of the projection scale factor which is based on your location on the projection and the elevation factor which is based on your current elevation. These factors combined are your combined ground to grid factor. You also need to be aware of the direction you are applying this factor, grid to ground or ground to grid. Ground been your current location which is what you get with the total station. I usually prefer to work in a local projection and use a local coordinate of my choosing at the base. Others prefer to use state and utm projections and go back and forth from grid to ground.
chris mills, post: 411136, member: 6244 wrote: Have you taken into consideration the height above sea level? The scale factor is correct only at sea level height and will need to be corrected to the height at which your are observing. Your software might be compensating for this on the GPS readings (since the GPS will be providing information on the height above datum) but not on the total station data.
Ahh okay.. So If I see a provincial control monument card with a scale factor on it, I'll need to worry about adding in the height factor or is it already taken care of?
Paul in PA, post: 411256, member: 236 wrote: Lastly I note that Mr Ewen MacDonald has not checked back in here since posting.
Hey Guys sorry about the radio silence here but I have been drawing up some red-lines, all good now.
Paul in PA, post: 411256, member: 236 wrote: "Static Shots" needs explanation, are these individual observations, static processed through a base station system, differential static post processed? Other than multiple receivers, too many questions, no answers.
I only had 2 receivers so I just set up the base over a known point and then tied in 2 other known control points and 3 new project points with my rover receiver on a tri-pod and then imported into the new Leica Infinity software, I have StarNet software as well but I can't remember how to use it great.
Paul in PA, post: 411256, member: 236 wrote: I do not know nor can speculate where this scale factor came from, but it is my impression it should not have been used.
That Scale factor just came from one of the known control points of which I found a control card on.. very close to the new barn but I haven't figured out why that is not a great scale factor to use. The coord system came off of the control card as well.
...And I do feel like a bit of an idiot now as the coord system was a utm coord system and not a 3tm, 3tm is something I use often at home in alberta so I brain-farted. Even upon using the coordinate system the previous surveyors were using I still have a discrepancy of around 20mm on a baseline of around 300m.
Upon asking a local surveyor, they said that they always use that coord system... However I had to guess which geoid model they were using and I feel that it might not be correct. Would the geoid affect anything other than elevations? (Stupid question I know, but I'm just trying to rule that out.)
I haven't post-processed or adjusted anything yet as I do not know whether to adjust the GPS baselines to the Total Station Data... Or the Total Station data to the GPS data (I'm thinking the latter)
Also a concern arrises where I have this discrepancy, it worries me to adjust everything together knowing that the points will have a lot of error to average out.
Much Appreciated guys!, The stakes are low for this project but I'm interested in learning this so I can apply it on other projects down the road perhaps (After heaps of study of course!)
Just to clarify a few points on the UK coordinate system.
Great Britain use a European derivative of WGS84 known as ETRS89. GB mapping is on a single TM projection which covers the whole of the country. Up until widespread use of GPS this projection was documented as OSGB36, a triangulated network adjusted (horribly in the case of outlying areas to the west of Scotland - from the scale factor given Mr.MacDonald's area appears to be somewhere on a line through Glasgow and Inverness so the adjustments shouldn't be too poor) to give the most reasonable fit to the populated areas of the country. The scale factor varies from 0.9996 down the central meridian, which is roughly along a line through Bristol and Newcastle, out to 1.0004 at the extremities of the mainland.
The Ordnance Survey list details of all the permanent stations originally installed as part of the network - the scale factor quoted by Mr. MacDonald will have been that calculated for the control station he used and listed with its coordinates. It will have been corrected for the height above mean sea level.
Up until 3 months ago conversion between ETRS89 and OSGB36 was by means of a transform OSTN02 (and the associated geoid model OSGM02) which adjusted true position on a kilometre square basis to get a better fit to OSGB36. The latest transform is OSTN15 (and OSGM15) which improves the accuracy of fit. The relative changes from one km. square to the next are very small so practically for a single small project it won't show.
Official guidance at:
https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/surveying.html
Ewen, I'm intrigued - how many surveyors go on holiday and take a full set of equipment with them? Wouldn't it have been easier just to hire gear locally for a couple of days rather than ship yours across the pond?
Chris has hit the nail on the head there. Let me now if you would like the OSGB36(15) coordinate system (inc. Geoid model) for Infinity or your kit.
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chris mills, post: 411475, member: 6244 wrote: Ewen, I'm intrigued - how many surveyors go on holiday and take a full set of equipment with them? Wouldn't it have been easier just to hire gear locally for a couple of days rather than ship yours across the pond?
Hey Chris, haha yes definitely cheaper to do so but as it's a family farm I wanted to create a bunch of maps and things from the farm also for personal use (I'm trying to make some old fashioned wall maps for my House in Canada. My brother has much of the survey gear I needed as he works in Edinburgh so I only needed to take the total station on the plane as hand luggage!
phil_shutt, post: 411959, member: 12458 wrote: Chris has hit the nail on the head there. Let me now if you would like the OSGB36(15) coordinate system (inc. Geoid model) for Infinity or your kit.
Hey Phil, That would be great if you could send me that coord system as it's been on my to-do list for a while! Cheers!
mrewenmacdonald, post: 412338, member: 11707 wrote: Hey Phil, That would be great if you could send me that coord system as it's been on my to-do list for a while! Cheers!
Have pm'd you with a link for download
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Good pure GPS vectors measure a length between two ends. Its subject to a little fuzz and such and quality of equipment used. The calc's as John Hamilton and others have referenced are mostly pure and should compare very closely with a properly staged EDM shot.
Where GPS goes weird is when projections, scale factors and other black box math is applied and the user doesn't understand it or set it up properly. IT'S A BIG BLACK BOX and almost ANYTHING CAN BE PRODUCED THERE. Projections are a great and valuable tool but much manipulation of the actual true GPS vectors is going on there. The math massaged numbers that come out may or may not be in tune with the actual raw distances from their GPS vectors.
Vectors: The actual slope straight line dimemsions, between the mathematical center of GPS reciever A, and GPS reciever B.
The mathematical center, is sometimes just above the top of the antenna.
Then, it is corrected down, to be at the point of measure, for antenna height. Think in terms of instrument edm offsets, or the -30 mm prism offset.
So, if the antenna, and pole are not totally plumb, it "adds more potential error source".
Of course, gravity, does not pull straight down. So, using a 2 meter pole, (or the same antenna heights ot both ends) tends to index out this source.
There's often more going on, with gps, than most end users realize.
N
