Every year, LSAW gives back to it's members with a free seminar. It's also a way to promote membership; especially associate members.:-D
This year it's our chapter's turn so I decided to provide RTK training!
I've been following all of the RTK threads here for inspiration. I would like to give special thanks to Kent; for being the antagonist that he is. We plan to address every problem you can have, using RTK. It would be a real treat to have Kent attend. Maybe we can start a crowd fund and buy him a ticket. :snarky:
The date has been set: September 25, 2015. All day and a free lunch! Leica, Trimble and Topcon will be there too.
Details to come.
Douglas Casement, PLS
SPSC-LSAW President
Hi,
Can you address the WGS84 Ellipsoidal Height value & why it can/can't be use to get elevation differences between points?
I read a lot about why GPS can't be used to get heights like we use levels for levelling because the values belong to different datums & yet we use RTK values for topography and bathymetry surveys that rely on differences in ellipsoidal heights as elevation values.
Also how far can you use RTK to get reliable results in hor & ver values. You get different claims from different manufacturers but their basic specs says vert is good to within 0.02 m + xxx/ppm. the xxx/ppm is the interesting part that most users often forget. When the DC says errors in h/v are 0.02 - does it includes the xxx/ppm? I would think so but if that is the case then the xxx/ppm seems to be negligible or is it?
Thanks.
WGS 84 geoid heights
At the bottom of this note are links to two web-based geoid height calculators for use with WGS 84 coordinates.
Without excessive complicating digressions, to derive an "orthometric" height (H) from an ellipsoid height (h) one needs the ellipsoid-geoid separation at the point (N).
Given any two of the three quantities (H,h, and N) one can solve for the third using: h - H - N = 0
Is an orthometric height determined this way compatible at the centimeter level with leveled heights? It depends on the height system's definition.
As you are in Singapore, I will not bore you with NAD83/ITRF/WGS84 differences.
Check with your country's surveying authority for details on their height system. Perhaps someone on this Board knows. I do not.
http://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/intptWhel.html
WGS 84 geoid heights
The geoidal separation can be easily determined here because there are a lot of MSL benchmarks that the GPS can occupy. So you get the values of the ellipsoidal height & the MSL/Orthometric values for comparison among several closeby benchmarks.
What I am confused about is the talk that you can't use RTK/Static GPS as replacement for differential leveling because you get different values due to datum differences. If RTK results are within 0.02 +xx/ppm then for a 10 km straight line the error would be less than ~0.03m. That in my opinion is better than running a loop for 20 km. Error in the loop level would be within or close to this RTK error.
Am I correct in my evaluation?
WGS 84 geoid heights
The geoid is the imagined "sea level" surface and it is lumpy. The distance between the geoid and the ellipsoid used by WGS84 can change by a few cm per km.
Converting GPS measurements to elevations above the geoid or sea level depends on knowing that variation of the geoid. If you have accurate enough knowledge (geoid model) then you can make the conversion.
WGS 84 geoid heights
Countries like the US and Canada are working to base their national height systems on GPS-derived ellipsoid heights and a high-resolution geoid model. These countries face the dilemma of updating and maintaining their height systems over vast areas. Some smaller countries have chosen the option of releveling. What is Singapore doing?
While you mention the accumulation of error as unbounded, a well designed network minimizes the accumulation of error. Well conducted differential leveling is more precise than GPS-based "leveling". Some of the defects of earlier national height systems were due to the absence of information now available from sensors like GRACE and GOCE.
An ellipsoid height is a rather challenging value to determine accurately. The challenges include antenna phase pattern variability (for which there are now good models), multipath (can be mitigated by hardware and software), but especially tropospheric effects. I refer to static GPS only as RTK techniques, IMO, are not sufficiently robust to provide good heights.
The relationship between h, H and N expressed as h - H - N = 0 does not acknowledge that there are errors in each value. These heights systems are also NOT parallel. I cannot assume that the ellipsoid-geoid separation at a nearby point is the same area. Depending on the geoid slope in an area even points separated by short distances could have different separations.
The resolution of geoid models remains an issue. Of course the resolution is improving with new data and modeling. I noted that the tools mentioned in my earlier message did not use the most recent Earth Gravitational Model (EGM 2008) which has degree and order 2159 but used EGM 96 with degree and order of 360. For information on EGM 2008 see:
http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/
Note as well that there are different types of geoid models. In the early 1990s hybrid models were developed in the US, Australia and elsewhere to better fit the relationship between the ellipsoid and orthometric heights. These hybrid models are based on gravimetric models like the WGS 84 model linked in my earlier message. U.S. hybrid models (having GEOID in their names) also include data transformations.
Does Singapore have a hybrid geoid model?
Hope this is responsive,
DMM
WGS 84 geoid heights
Franz,
If there are a lot of GPS on BMs in Singapore than the mapping authority should have the data needed to compute a hybrid geoid model that would provide reasonable high quality orthometric heights in the national vertical datum. This is exactly what the U.S. National Geodetic Survey (NGS) has done. You might want to look at NGS publications NOAA TM NOS NGS-58 "GUIDELINES FOR ESTABLISHING GPS-DERIVED ELLIPSOID HEIGHTS (STANDARDS 2 AND 5 CM)" and NOAA TM NOS NGS-59 "GUIDELINES FOR ESTABLISHING GPS-DERIVED ORTHOMETRIC HEIGHTS". In areas of the US where there are lots of GPS on BMs to support the model surveyors can typically achieve 2-4 cm level heights without much difficulty.
RADAR, post: 321240, member: 413 wrote: Every year, LSAW gives back to it's members with a free seminar. It's also a way to promote membership; especially associate members.
Does that mean I don't have to be an LSAW member to attend? Thanks!
Zapper, post: 322205, member: 6470 wrote: Does that mean I don't have to be an LSAW member to attend? Thanks!
No, you don't have to be a member, but then it won't free :'(
The cost is the same as a membership and it's a big savings on an associate membership.
Feel free to contact me if you need anymore information.
Dougie
Thanks Radar. I may just look into the associate membership! B-)
No Zapper; Thank You! LSAW is a great organization and we are always looking to grow! Together; is the only way we will ever have a positive impact on our communities.
Please let me know if you ever need anything or have any questions or concerns.
Thank you,
Douglas Casement, PLS
:good:B-)
When Kent goes rtk, it'll be march, of 2060. When a 2 min observation will give you millimeter (McMillimeter) accuracy.
When Kent goes rtk, it'll be march, of 2060. When a 2 min observation will give you millimeter (McMillimeter) accuracy.
