[time-nuts] China, GCJ-02 & cartography

Eric Scace eric at scace.org
Tue May 15 14:11:37 EDT 2018

I live at the edge of Boston Harbor, in the Charlestown Navy Yard.

Tide stations, such as this one at Boston Harbor’s Fort Point Channel <https://tidesandcurrents.noaa.gov/stationhome.html?id=8443970>, report their tides against a locally-defined datum <https://tidesandcurrents.noaa.gov/datums.html?id=8443970> of mean lower low water (MLLW, the average of the lower of the two low tides reported each day, for all days observed in the current national tidal datum epoch of 1983-2001).

For this station, MLLW = +5.51 ft elevation, referenced to NAVD88.

There is a National Geodetic Survey benchmark <https://www.ngs.noaa.gov/cgi-bin/get_image.prl?PROCESSING=get_figure&IID=63035> close to the tide gage. <https://www.ngs.noaa.gov/cgi-bin/get_image.prl?PROCESSING=get_figure&IID=63034>

The  benchmark record <https://www.ngs.noaa.gov/cgi-bin/ds2.prl?retrieval_type=by_pid&PID=MY0555> states:
MY0555                         *CURRENT SURVEY CONTROL
 MY0555  ______________________________________________________________________
 MY0555* NAD 83(1986) POSITION- 42 21 18.1    (N) 071 03 03.4    (W)   HD_HELD2
 MY0555* NAVD 88 <https://www.ngs.noaa.gov/datums/vertical/VerticalDatums.shtml#NAVD88> ORTHO HEIGHT -     4.105 (meters)       13.47  (feet) ADJUSTED
 MY0555  ______________________________________________________________________
 MY0555  GEOID HEIGHT    -        -27.738 (meters)                     GEOID12B
 MY0555  DYNAMIC HEIGHT  -          4.104 (meters)       13.46  (feet) COMP
 MY0555  MODELED GRAVITY -    980,381.5   (mgal)                       NAVD 88
 MY0555  VERT ORDER      -  FIRST     CLASS II

As I understand it, here comes the messy part of getting from NAVD88 geopotential geoid to WGS84 ellipsoid. I used NOAA's on-line vertical data transformation tool <https://vdatum.noaa.gov/vdatumweb/?dump_app_trace=false&db_debug=false>, using the 2009 May 16 benchmark recovery date specified in its record.
long = -71.0509444
lat = +42.355028
NAVD88 height = +4.105 m

The WGS84 result was:
long = -71.0509458
lat = 42.3550371
height = -24.850 m ±0.076m

   If I did this correctly, to convert from NAVD88 height to WGS84 elevation at this location requires subtracting 28.954m. The MLLW datum of +5.51 ft (+1.679m) becomes -27.275m in WGS84.

   The benchmark record above states that the 4.105m NAVD88 height is -27.738m height in GEOID12B at this location. One could double check the NAVD88 → WGS84 conversion by starting with the GEOID12B height. Sadly, GEOID12B does not appear to be an option for the vertical data transformation tool that I used.

   But I’m a complete novice at these transformations.

— Eric

p.s.: To confuse matter further, at the moment my iPhone’s Compass tool reports my current height as "50 feet”. I don’t know what is the basis for that determination, but it doesn’t seem correct for a NAVD88 or WGS84 height.

> On 2018 May 15, at 12:13 , Bob kb8tq <kb8tq at n1k.org> wrote:
> Running with a very normal WGS-84 GPS “by the sea shore” can easily show you underwater. That
> is very much a normal result of the model. It does not tell you what high (or low) tide level is going to
> be at your location. That stuff is simply to complex.

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