PPK - Finding Base Position

Reach Module / M+ / M2 RTK / PPK configuration
22

GNSS/GEOID heights are really a messy subject. Without going into much detail my guess would be the height I came up with in RTKLIB is biased about a decimeter (4 inches) too high. This is because atmospheric errors aren’t canceling that well. Using the Iono/Tropo options Estimate TEC and Estimate ZTD RTKLIB can adjust for this, but then fixing the integers is more difficult.

Unless you really want to dig into the weeds it’s probably best to just send your data to a professional (free) service like CSRS PPP, OPUS, or AUSPOS.

2 Likes
23

Yep… just what we discussed in other thread.

Orthometric GNSS derived versus ground.

1 Like
24

Thanks again. I am confident the point is within 2-3cm horizontally and am not concerned with the vertical as it is coming from ground benchmarks being leveled in. Fortunately this scenario is the exception instead of the rule. State Plane coordinates were off bad enough that I couldn’t even process with GCP’s. Everything was about 1000ft NNE and that doesn’t jive with the scale factor so I don’t know what was done prior to us receiving it that caused the shift. Instead of using a custom conversion I decided it would just be easier to shoot it in reality and do all the alignment in CAD after processing.

25

Thanks! Now the Elipsoid elevation is correct again.
Getting this coordinate: x=958977.559 m y=3099883.165 m y=190.750 m, stdev 0.003 0.003 0.005

Using these stations combined:
lcsm
sam2
txc2
txsm
txta

What Geoid is used? NAVD88 ? GEOID18, what Grid number? What is it called over there?
Overall, it would really interesting, given our recent discussions, if you could come back with the height from local monuments, so we can compare theory vs real-life.

1 Like
26

Geoid18 is now the reference. If you have a true ellipsoid height you’re confident in you can go here to compute the the geoidal speration

https://geodesy.noaa.gov/GEOID/GEOID18/computation.html

At least for published updated datasheets. This will give the orthometric heights

27

Correct we use Geoid18, but I don’t know what the surveyors that provided the control used.

1 Like
28

Yep same problems we face tracing others. We literally have to resurvey others to determine what they did. Calling them and discussing leads to another dimension.

29

I double checked my setup in field genius and I used geoid 12b grid 6. When adjusting to the level elevations there was an average of about 3.5-4 feet. I thought feel genius was reporting orthometric heights, but I think that might only be the case when I localize and not just shooting off the geoid. I will verify with them that there’s not something in the software that is creating orthometric heights from the geoid.

1 Like
30

Roger… so, run it all again using:

-----------------------------------------------------------------------------
|  Network              |  Network 1                                        |
|  Project              |  New Project                                      |
|  Adjustment Date      |  2020/11/19 12:19:47.99 (UTC)                     |
|  Mapping System       |  SPCS NAD83(2011) 4203 Texas (Central)            |
|  Projection Template  |  Lambert Conformal Conic                          |
|  Datum                |  NAD83 - National Spatial Reference System (USA)  |
|  Geoid Model          |  CONUS Grid #0                                    |
|  Units                |  m                                                |
-----------------------------------------------------------------------------

Final coords:

ADJUSTED COORDINATES
------------------------------------------------------------------------------

    RAW_20201117
     WGS84                              m    NAD83 - National Spatial Refe***m
     Lat:  N 30 32 21.64404    +/-  0.002    Lat:  N 30 32 21.62916 +/-  0.002
     Lon:  W 97 38 02.57159    +/-  0.002    Lon:  W 97 38 02.53190 +/-  0.002

     Elevation (m)                           Elevation (m)
     Hgt:  189.505             +/-  0.003    Hgt:  190.746          +/-  0.003
     Und:  -25.974                           Und:  -25.974
     MSL:  215.479                           MSL:  216.720

                                             SPCS NAD83(2011) 4203 Texa*** (m)
                                             X:    958977.559       +/-  0.002
                                             Y:    3099883.165      +/-  0.002

Loops:

---------------------------------------
|  Project            |  New Project  |
|  Segments per loop  |  2            |
|.....................|...............|
|  Number of loops    |  6            |
---------------------------------------

Statistics for ALL loops
-------------------------------------------------------------
|                |              |  ___Closure___  |         |
|                |      Length  |     2D       H  |    PPM  |
|                |         (m)  |    (m)     (m)  |         |
|----------------+--------------+-----------------+---------|
|  Best          |              |  0.000  -0.000  |  0.027  |
|  Worst         |              |  0.009  -0.006  |  0.150  |
|  Average loop  |  104123.952  |  0.005   0.004  |  0.071  |
-------------------------------------------------------------

Undalation between the Geoid model in EzSurv and the link above match to the millimeter :smiley:

A final screenshot with a more visual approach to the stations used (closest 18 km, farthest 78 km):

MapStations
MapStations988×1156 5.4 KB

Now it is just how it all match reality.

3 Likes
31

You the man Christian ! That’s what I do too ! Sometimes difference between orthometric and ground is nill.

But be careful, it’s not always the case.

Excellent processing !! You the PP man now !

1 Like
32

Awesome, but the EPSG we use is 6578. Lambert conformal… I guess I was meant to map!

33

To have it in ft, and not in meters, right?

34

Different types of feet, ift (international feet) and usft (US feet). I’m not sure what Texas uses, is it USFT ?

1 Like
35

Yes, survey feet. (ftUS) Thanks!

For those not familiar it’s survey feet because it is decimal tenths of a foot instead of the silly fractions others use…

36 
-----------------------------------------------------------------------------
|  Project              |  W:\temp\ForumChascoBase\EzSurv\Chasco.spr        |
|  Geoid Model          |  CONUS Grid #0                                    |
|  Mapping System       |  StWS NAD83(2011) Texas Centric Lambert           |
|  Projection Template  |  Lambert Conformal Conic                          |
|  Datum                |  NAD83 - National Spatial Reference System (USA)  |
-----------------------------------------------------------------------------

Adjusted Results (from Least Squares)
-----------------------------------------------------------------------------------------------------------------
|                |  __________________Position_________________  |  _Standard deviation_  |                     |
|  Site          |            X             Y   EllHgt      MSL  |      X      Y     Hgt  |  Source   Provider  |
|                |         (ft)          (ft)     (ft)     (ft)  |   (ft)   (ft)    (ft)  |                     |
|----------------+-----------------------------------------------+------------------------+---------------------|
|  RAW_20201117  |  5664542.040  21001194.142  625.807  711.024  |  0.006  0.005   0.009  |  Network  N/A       |
-----------------------------------------------------------------------------------------------------------------
37

That’s interesting. Our State Plane coordinates are 3M,10M (X,Y) range. What CRS is this?

Now we see how whack our stuff is… :rofl:

38

Here we go:

-----------------------------------------------------------------------------
|  Project              |  W:\temp\ForumChascoBase\EzSurv\Chasco.spr        |
|  Geoid Model          |  CONUS Grid #0                                    |
|  Mapping System       |  SPCS NAD83(2011) 4203 Texas (Central)            |
|  Projection Template  |  Lambert Conformal Conic                          |
|  Datum                |  NAD83 - National Spatial Reference System (USA)  |
-----------------------------------------------------------------------------

Adjusted Results (from Least Squares)
-----------------------------------------------------------------------------------------------------------------
|                |  __________________Position_________________  |  _Standard deviation_  |                     |
|  Site          |            X             Y   EllHgt      MSL  |      X      Y     Hgt  |  Source   Provider  |
|                |         (ft)          (ft)     (ft)     (ft)  |   (ft)   (ft)    (ft)  |                     |
|----------------+-----------------------------------------------+------------------------+---------------------|
|  RAW_20201117  |  3146245.538  10170200.005  625.807  711.024  |  0.006  0.005   0.009  |  Network  N/A       |
-----------------------------------------------------------------------------------------------------------------
1 Like
39

Don’t get me started on this. Our local Modified Transverse Mercator system is metric, because yay modernity. The central meridian for each zone has a false easting of 304800, because why pick a normal number when you can just translate the old system’s value of 1 000 000 feet into meters. :crazy_face:

40

ift (1 foot) = 0.3048 meters exactly

usft (1 foot) = 0.3048006096 meters

Makes a big difference using large coordinate values if using wrong foot.

SC is ift (international foot)

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41

Literally starting off on the wrong foot… I just hope I did my meditations that morning.

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