Do you need to measure up Everytime when you use gps even if you are not doing elevations
> Do you need to measure up Everytime when you use gps even if you are not doing elevations
It's usually a good idea; who knows when a client may come back and say:
"We didn't need grades before, we need them now. You can provide that to us easily right?"
It simply prevents a return trip to the point you burned to determine it's elevation.
I believe there are also technical reasons why a measure up is recommended that have to do with post-processing, but I can't recall what the reasons are at this moment.
No.... but there are alot of things going on. And things that go wrong without an accurate HI & elev. Especially with Post processed networks.
I suggest you KEEP asking and thinking. Gotta run.
N
“NEED” to?
Probably not, it shouldn't change your N/E position much one way or the other. Bear in mind that GPS “works” in geocentric X/Y/Z (NOT to be confused with local horizon N/E/U), so you are kinda...well...I dunno, it just seems WRONG to me.
Personally, I don't really understand why anybody would chose NOT to, but that's just me. As far as “elevations” go, that's the job of a LEVEL, not GPS.
I have found GPS (Static & RTK) to be very good at “approximate elevations” (+/- 0.1m absolute), and usually pretty good at relative elevations differences on small projects (subject of course to the GEOID Model precision in a given area).
Loyal
Yes
The way it was explained to me is that the HI & HR will impact the plane you are working on, although not greatly. GPS is technically radial surveying, so imagine the 3D triangle it solves for each measurement. The satellites are up there somewhere, but your rover/base is where you are. Without an accurate HI or HR, the data plane where you are will not be as close as it could be. Thus when you inverse between data points on your plane (project), you'll introduce the vertical inaccuracies.
Better to put 5.0 or 6.0 than leave it as 0.0.
> Do you need to measure up Everytime when you use gps even if you are not doing elevations
Nope. As Loyal suggested, putting in a zero (or other small value) antenna height will not have a measurable effect upon the computed horizontal position of a point. However, as others have pointed out, there doesn't seem to be much justification for not putting in a reasonably accurate antenna height, since it doesn't require much effort to do so.
If I had a network for which I only needed horizontal, I'd still be measuring antenna heights. (I rarely use anything other than fixed-height tripods for GPS work, so antenna height just comes with the territory.) But if I had a setup for which I forgot to record a height, I'd probably use zero when processing and make copious notes in the field book, job file and any associated drawing files that the height of that point is not correct.
YES! It is a three dimensional system. For the computations to be correct, all three unkowns (lat, long, height) need to be solved for.
> Without an accurate HI or HR, the data plane where you are will not be as close as it could be.
As I understand it, GPS processors determine the XYZ position of the antenna phase center. Since most of us want ellipsoid coordinates (lat/long), the software converts the XYZ to ellipsoid values, and then subtracts the antenna height from the ellipsoid height to arrive at the ground station ellipsoid height. (Then it applies a geoid separation if one is requested and available to get the orthometric height of the ground station.) So it makes no difference to the processor whether the antenna height is 0 meters or 2 meters or 100 meters, the ellipsoid latitude and longitude of the ground station will be the same as that of the antenna phase center.
In my opinion, absolutely yes. Please recall that elevation affects scale factor thus horizontal position during projection process.
We need to keep in mind here, that what is really being “solved” can be best described as a 3-dimensional vector (dX/dY/dZ) from L1 phase center (L1pc) to L1 Phase Center. Now we could (I suppose) argue the [relative] position v. vector (chicken or the egg) analogy, but every commercial software product that I have dealt with, RETURNS a dX/dY/dZ vector either L1pc to L1pc (RTK) or Mark-Mark (Static).
When you then solve the resulting X/Y/Z to Lat/Lon/Eh, the “error” introduced by NOT having the correct “HI/Rod” is NOT going to impact the Lat/Lon enough to worry too much about. Your height will suck, but that doesn't alter your Lat/Lon unless your rod/setup is pretty out of kilter, in which case the Lat/Lon would suck anyway.
SHOULD you “measure up?” I think so, but it's not really going to degrade your Lat/Lon, SPC, UTM, whatever, very much. Do the math, and you'll see what I mean.
Loyal
In a flat world questions like this don't come up. In the round world of GPS, the question does. It would seem that understanding the concept of the GPS'ed 3D triangles should be pretty straightforward, and I think that was the original post. It is important. Remember that the option of software adjustments and manipulation do not appeal to many, myself included.
There are a great many of us surveyors who calibrate their projects, and not work on grid. Many others don't understand why in the world we would ever do that. Kind of like many use GPS as just another tool in the truck, while some think it is the only tool in the truck.
$0.02
YES... dont you like one word answers???
If you are talking about RTK, I try (emphasis on try) to keep track of the elevations even if it is strictly a horizontal project. Or, if the project does involve contours, I try to keep track of the elevations because all those extra shots are often helpful in generating contours.
I use a slide pole for most of my GPS work, and the last thing I want to do is get in the habit of running the pole up or down without noting the change in rod height. There is nothing like taking a bunch of shots and suddenly remembering you ran the rod up several feet about fifty shots previous. And there is nobody to blame but me because I'm the party chief.
I keep track of the HI and throw it into the mix primarily because of the software but pay little attention to the elevation results. When I hold a known location, I only hold the horiz position, not elevation. Don't think 5 to 8 feet will have enough of an impact on the combo factor to be even considering that part of the results.
jud
It takes so little extra effort and time to log the data that it's worth doing, as others have said you just don't know when you might need it. It's not just elevations at points, but converting from grid to ground, sea level to terrain level etc, all of which benefit from knowledge of heights for accurate scale factors.
gps measure up = antenna height
YES
Antenna height affects both horizontal and vertical, as related to gravity, measurements.
The error in ECEF position in a radial direction from the polar axis is a function of the product of the antenna height and the cosine of the latitude. The error is then propagated in the ECEF cartesian coordinate system as a function of the cosine of the longitude in the "x-direction" and a function of the sine of the longitude in the "y-direction." In terms of the ECEF cartesian coordinate system, the "mark" positional determination is incorrect if the proper antenna height is not accounted. In relative positioning, as long as the antenna heights are the same for every observation, then there would be no problem over a small area. Because state plane coordinates and other projections are a function of the ECEF cartesian coordinate system, accurate antenna heights are required for accurate projection coordinates.
Errors as a function of antenna height are least at the earth's poles and greatest at the equator ie cos(90) = 0 and cos(0) = 1.
gps measure up = antenna height
I always measure up, regardless if it's GPS or TS. Like stated earlier, you never know when you will need the elevations!
-JD-
gps measure up = antenna height
> In terms of the ECEF cartesian coordinate system, the "mark" positional determination is incorrect if the proper antenna height is not accounted.
Agreed. However, the ellipsoidal latitude and longitude will be correct within the limits of the equipment and procedures employed, and if an accurate antenna height were known the correct ECEF position could be recreated from the ellipsoidal values.
The OP asked whether antenna heights were necessary "if you are not doing elevations." From this I inferred that he wanted to know if he could rely upon the horizontal positions (likely projected to SPC) for common land surveying purposes. I maintain that the errors introduced in horizontal position by zeroing or guesstimating antenna heights are well below the noise level of the technology. Sloppy practice? Sure. Harmful? Nope.
gps measure up = antenna height
> > In terms of the ECEF cartesian coordinate system, the "mark" positional determination is incorrect if the proper antenna height is not accounted.
>
> Agreed. However, the ellipsoidal latitude and longitude will be correct within the limits of the equipment and procedures employed, and if an accurate antenna height were known the correct ECEF position could be recreated from the ellipsoidal values.
>
> The OP asked whether antenna heights were necessary "if you are not doing elevations." From this I inferred that he wanted to know if he could rely upon the horizontal positions (likely projected to SPC) for common land surveying purposes. I maintain that the errors introduced in horizontal position by zeroing or guesstimating antenna heights are well below the noise level of the technology. Sloppy practice? Sure. Harmful? Nope.
I agree with Jim in that the OP asked if antennae heights were necessary when not doing elevations. Unless I'm missing something (which I probably am), by not measuring HT on a 2 meter pole you would introduce a systematic error on the magnitude of 0.3 ppms. Hardly something to worry about.
Ralph
