JOHN NOLTON, post: 420254, member: 225 wrote: Above in GeeOddMike post (note: when i looked at post by this person I always thought it was " Gee Old Mike" till I put on my close- up glasses) the nice picture is not correct---- at least for the conterminous United States.
Why don't they show a correct drawing of the relationship of the ellipsoid, ground and geoid. That should not be asking to much from NGS. The math is very simple but a correct picture is worth a 1000 words.
** In the conterminous United States the ELLIPSOID is ABOVE the Geoid. In Alaska the ELLIPSOID is BELOW the Geoid.JOHN NOLTON
You are correct that the the drawing shows one of two ellipsoid-geodesic relationships. The point was in either case you need the separation.
Generically h-H-N=0
BTW, isn't Alaska part of the US?
I am sorry but I don't get any of your problems with those geoid separations diagrams. I have been using base/rover setups for more than a decade in my bathymetry work. I just place the base over a BM with either MSL or MLLW elevation and my rover in the boat together with the echo sounder setup. Both are either in RTK or PPK mode depending on how far it is from the shoreline or availability of radio/cell signal.
When I process the data, I use WGS84 datum and I just replace the elevation to reflect the BM's elevation (MSL/MLLW) and I will get same datum results from my rover data.
No messing around with geoidal height separations etc.
I think you are a bit confused between ellipsoidal heights & geoidal heights.
FrancisH, post: 419576, member: 10211 wrote: RTK has a vector limit of ~5km?10km? depending on how good your radios/cell signals are between work & benchmark sites.
If distance is too far away for RTK to work then you revert to static which is 1 hour?4 hours? repeated sessions? That in itself is
1 whole day going to base/rover sites. What are your errors?1cm? You must be kidding me. Repeated static sessions have
accuracies of 2-3 cm. vertical. That is the instrument inherent accuracy. It means with the best satellite orientation available
and no obstructions then you are lucky to get within this range.
In 4 hours you could run a 5 km loop and your results would be within 1st/2nd order if you will be using a digital level.
20 or so years ago we saw 2 to 4 centimeters on extended static observations. These days i routinely get closer to 2 to 4 millimeters using dual base rapid static techniques. Two guys can blanket half a county in one long day if they have a plan. Three guys would still be shuttling trucks at the end of a week.
FrancisH, post: 420324, member: 10211 wrote: I think you are a bit confused between ellipsoidal heights & geoidal heights.
I nominate your post for Best Comedic Posting of 2017. Suggesting that GeeOddMike is confused about any matter geodetic he might choose to post about is like saying that Einstein was confused about physics. Apparently you do not know of whom you speak.
Perhaps in Indonesia there is no good geoid model available, and you have to get on as best you can. I don't know. Sure, when topo-ing the bottom of a body of water local variation in geoidal seperation is not likely to be a significant source of error. But, jeesh, ROTFLMAO.
FrancisH, post: 420324, member: 10211 wrote: No messing around with geoidal height separations etc.
Assuming you want orthometric heights, which is what most work in the US is tied to, then you need to consider the geoid slope in the area you are working in, the distance from the reference bench mark, and your error tolerance for the project. As an example, see the numbers I posted earlier for the area I'm in.
For bathymetric work, maybe it isn't critical. For careful work in an area with a large geoid slope, you couldn't go very far before accumulating a significant error in orthometric elevation, because the difference (ortho - GNSS) changes with location.
Mark Mayer, post: 420328, member: 424 wrote: I nominate your post for Best Comedic Posting of 2017. Suggesting that GeeOddMike is confused about any matter geodetic he might choose to post about is like saying that Einstein was confused about physics. Apparently you do not know of whom you speak.
Perhaps in Indonesia there is no good geoid model available, and you have to get on as best you can. I don't know. Sure, when topo-ing the bottom of a body of water local variation in geoidal seperation is not likely to be a significant source of error. But, jeesh, ROTFLMAO.
Seconded.
FrancisH, post: 419137, member: 10211 wrote: hey,tell you what,let's just get a topographic map off the USGS site for your client's site and save him a lot more money.
costs are costs, you either pay them to get what you need. you get what you paid for... as the adage goes.
A super on a construction site tried to blame us for doing just that after a 300' long by 8' tall precast block wall fell nearly a foot low to the parking lot grade. Turns out they (the installer) was reading decimal feet as inches.
I nominate your post for Best Comedic Posting of 2017. Suggesting that GeeOddMike is confused about any matter geodetic he might choose to post about is like saying that Einstein was confused about physics. Apparently you do not know of whom you speak.
Hahahahahahahahhahahahahahahahahhahahahahahah
That brought tears to my eyes. Hahahahahahhahahahahahha
There I go again.
Tell you what, why don't you get out whatever postprocessing software you are using and do some trial with your base/rover.
Place you base & set it to record static at 1 sec interval, same with your rover.
Place rover on rod & walk around an open field.
Process your data with your rover classified as dynamic.
Use first WGS84 for your vertical datum.
Change WGS84 to orthometric height datum.
Compare if the difference between all the points is constant.
Get back to me.
Hahahahahahahhahahahahahahahahahhahahaha
FrancisH, post: 420347, member: 10211 wrote: Hahahahahahahahhahahahahahahahahhahahahahahah
That brought tears to my eyes. Hahahahahahhahahahahahha
There I go again.Tell you what, why don't you get out whatever postprocessing software you are using and do some trial with your base/rover.
Place you base & set it to record static at 1 sec interval, same with your rover.
Place rover on rod & walk around an open field.
Process your data with your rover classified as dynamic.
Use first WGS84 for your vertical datum.
Change WGS84 to orthometric height datum.
Compare if the difference between all the points is constant.
Get back to me.Hahahahahahahhahahahahahahahahahhahahaha
The elevation of the base point will be correct, then as points are rayed out from the base they will slip away from the correct elevations by the difference in geoid heights, wouldn't be a huge issue, but I would see a foot in a couple of miles as I survey west from my office base.
20 or so years ago we saw 2 to 4 centimeters on extended static observations. These days i routinely get closer to 2 to 4 millimeters using dual base rapid static techniques.
hahahahahahhahaha
I use static results to orient my survey azimuth to plane after transformation. I observe 2 intervisiblestations (~100m apart).
Most of the time I compare the distance - hor & vert -with GPS results. Most of the time it gets ~1-2 cm (hor) but to say you are off by 2-4 mm is
ridiculous. The circular mark in your optical sight in your triback already covers ~2mm? Your nail on the forsight has a diameter of ~2mm?
That in itself covers your 2-4 mm intrinsic error.
FrancisH, post: 419620, member: 10211 wrote: really? the error brought about by just levelling your base/rover setups and getting your HI using a tape would introduce an error or what? 5mm??
how many times have you measured an HI and repeated it and then made your own reading at where the tape curls up when you placed it over the rod?the curled up portion already has several mm of inherent error.
We use fixed height tripods for bases and on static campaigns.
Well...I don't know who said it first, but he was right:
"YOU CAN'T FIX STUPID."
Loyal
And apparently you can't shut stupid up either.
Having been traveling most of the day, I did not get a change to either respond or research issues related to tidal heights and their relationship to other systems. This is an interesting topic.
After reading our Indonesia-based contributor's posts, I realized he is likely seeking to entertain himself and not actually contribute to discussion. I am happy to choose to ignore him using this site's helpful tools.
I will have to dig through the FIG site for more information about how nations like Indonesia deal with horizontal and vertical datum issues. Anyone have any recommended resources? I am aware of how the NGS and CO-OPS have tried to correlate their data. I may get something together to share.
As for the Odd v Old in my username, I am getting older; don't think any odder. The username was devised in a fit of whimsy.
Having been traveling most of the day, I did not get a change to either respond or research issues related to tidal heights and their relationship to other systems. This is an interesting topic.
Not sure what tidal heights has to do with the discussion. We were discussing using GNSS in vertical control. Then all of the sudden people were chiming in on effects of geoidal separations. Jeeez, the gps receivers give us difference in vertical values via default WGS84 ellipsoidal height.
This difference is almost similar to what you get using a TS. Not identical because results are based on satellites' signal qualities. But even how good they are, they haven't gotten as good as a level run or a TS vertical measurement.
To transfer your BM's elevation (whether it's MSL,ortho,MLLW,etc) you use WGS84 ellipsoidal values. You place your base on a BM with known height values and transfer to your control using rover's ellipsoidal ht results. You don't need to know all those geoidal separation and what have you.
People, the GPS receivers DO NOT KNOW where they are. It's up to you to put the base's elevation value in the DC or PP software.
This is how bathymetry surveys are done using RTK/PPK + sounder equipment.
Jeeez, all these surveyors in here and not one even understood ellipsoidal concept in gps receivers.
FrancisH, post: 420392, member: 10211 wrote: Jeeez, all these surveyors in here and not one even understood ellipsoidal concept in gps receivers.
TDD does, and he doesn't need to stop for water either.
Ìà
FrancisH, post: 420392, member: 10211 wrote: Not sure what tidal heights has to do with the discussion. We were discussing using GNSS in vertical control. Then all of the sudden people were chiming in on effects of geoidal separations. Jeeez, the gps receivers give us difference in vertical values via default WGS84 ellipsoidal height.
This difference is almost similar to what you get using a TS. Not identical because results are based on satellites' signal qualities. But even how good they are, they haven't gotten as good as a level run or a TS vertical measurement.To transfer your BM's elevation (whether it's MSL,ortho,MLLW,etc) you use WGS84 ellipsoidal values. You place your base on a BM with known height values and transfer to your control using rover's ellipsoidal ht results. You don't need to know all those geoidal separation and what have you.
People, the GPS receivers DO NOT KNOW where they are. It's up to you to put the base's elevation value in the DC or PP software.
This is how bathymetry surveys are done using RTK/PPK + sounder equipment.
Jeeez, all these surveyors in here and not one even understood ellipsoidal concept in gps receivers.
I would say most of us understand the ellipsoidal concept, additionally we understand the geoid model concept also. Without it GPS can't return elevations (unless you calibrate, which has lots of issues), clearly shifting the ellipsoid up or down to the elevation at one point doesn't work.
Francis H has demonstrated a clear lack of understanding of heights. You absolutely need some kind of geoid information to deal with hydraulic flow. This might mean a local geoid model, national, world (EGM08), or just hitting a few BM's surrounding the site and shifting and tilting the plane.
Yes, in a small area one can just use ellipsoidal heights, but that won't give very good results beyond a km or two.
And, when using a TS, the observations are with respect to the local gravity vertical (as is leveling), which is NOT the same as the ellipsoidal normal.
Advice to FrancisH: buy a good book on geodesy and read it before saying things that are not factually correct.
You all have been sucked into a battle of wits with an unarmed opponent.....again.
I nominate your post for Best Comedic Posting of 2017.
I agree, but I would vote for it all the way back to when I joined this forum, which was many years before the date shown in my profile.