Anyone know a trusted site for the download??ÿ error on the USACE page.?ÿ The areas i work still use NGVD29 for the vertical elevation certificates.?ÿ
Is there something you need that Corpscon does but the NGS Coordinate Conversion and Transformation Tool (NCAT)?ÿdoesn't?
from vdatum
"Important: Transformation Uncertainties in the ??Louisiana/Mississippi - Eastern Louisiana to Mississippi Sound?? Regional Model, have been found to range from 20 to 50 cm in particular locations from the Mississippi River Delta north to Lake Pontchartrain. These issues most likely can be attributed to subsidence, newly established datums, and changes to the understanding of NAVD88 based on new versions of the GEOID. The VDatum Team is currently looking at resolving these uncertainties."
?ÿ
?ÿ
NCAT- 1.0 ft NAVD88 = 2.35 ft NGVD29
Vertcon - 1 NAVD88 = 1.148 NGVD29
Vdatum - 1 NAVD88 = 2.057 NGVD29, Vertical Uncertainty (+/-): 18.78829 cm
If I need to convert an elevation from one datum to another I prefer to go find NGS datasheets for 3 control monuments surrounding the site which have elevations on both datums and compute an average offset to apply. That, basically, is what these programmatic solutions are doing anyway.?ÿ ?ÿ ?ÿ?ÿ
I'm not sure what's going on here, 'cause I'm getting very different results from NCAT.
NCAT works in meters only and only 3 decimal places, so we can't enter 0.3048 meters as one foot.
But think about NAVD88 vs NGVD29. Besides different ellipsoids, NGVD29 assumes that the ellipsoid height is always zero. I picked an easy point in NC (36.0N. 80.0W), entered assumed orthometric heights in meters and got the following results:
NAVD88?ÿ ?ÿ ?ÿ ?ÿ ?ÿ NGVED29
?ÿ ?ÿ 1.000?ÿ ?ÿ ?ÿ ?ÿ ?ÿ ?ÿ 1.226
?ÿ ?ÿ10.000?ÿ ?ÿ ?ÿ ?ÿ ?ÿ 10.226
?ÿ 100.000?ÿ ?ÿ ?ÿ ?ÿ 100.226
1000.000?ÿ ?ÿ ?ÿ 1000.226
So, at this point, any NAVD88 height is 22.6 centimeters (0.74 ft) lower than the NGVD29 height. That's not consistent with your results, so maybe I did something wrong.
?ÿ
A couple of comments.?ÿ First, Mark Huber who was the primary author of CORPSCON has retired and the Corps of Engineers is no longer supporting any updates to the utility so everyone who needs conversions or transformations should be using NCAT/VDatum.?ÿ The NCAT/VDATUM frames will be the host for the coordinate conversions and datum transformations for the new datums in 2024/25.?ÿ The other is that neither NGVD 29 or NAVD 88 or any other classical vertical datum in the world are referenced to a ellipsoid. Classical vertical datums are defined by leveling ties to one or more long-term tide stations.?ÿ
The only place the ellipsoid currently plays a role is in the determination of NAD 83 ellipsoid heights, which when combined with a geoid model does a relatively good job of estimating NAVD 88 or any of the US island vertical datum heights.?ÿ That will certainly change with NAPGD 2022 which will be based purely on a gavity based geoid model combined with the ellipsoid height (GRS80) as part of the structure of NATRF 2022.?ÿ The concept of the ellipsoid height being zero as noted by MathTeacher more appropriately applies to classical triangulation datums such as USSD, NAD, and NAD 27 in which the origin station (MEADES RANCH 1891 -KG0640) was considered to be at a point where the ellipsoid height is zero, which therefore minimizes the geoid separation at virtually any other location in the area of the datum.?ÿ At least that's how is was considered before NAD 27 was extended into Alaska.?ÿ As cordgrass indicated above using any NGVD 29 transformations in areas of known subsidence should be done with great caution and be HIGHLY suspect.?ÿ The '29 heights used in VERTCON were those published at the time of the NAVD 88 national adjustment, and in many, many cases those values would have been from leveling that was likely performed at least 20 to 30 years prior.?ÿ Consequently they did not reflect what should have been the contemporary height at the time of the '88 solution.?ÿ Since cordgrass did not provide the geographic location he's dealing with it's not possible to compare any values that would come from VERTCON/NCAT/VDatum.
?ÿ
NGS has suppressed the NGVD29 data in south LA due to subsidence.?ÿ It's hard to find a control point with NAVD88 data.
Thanks for the correction and additional information. My shorthand description, though not rigorously accurate, helps me keep things orderly from one day to the next.?ÿ
To your point, when I did the quick look at an NC point in NCAT, using height with NGVD29 causes NCAT to put its little locator flag at Meades Ranch on the map. I actually thought that Cordgrass lives in North Carolina. I guess I'm localizing everything as I get older.
Sorry for the insult, Cordgrass.
Models are models and survey is surveying. One is better than the other.
That's interesting, typically I use a FIRM map that has bench marks listed as control. Then of course the bench marks are located and checked against Geoid models to find how they actually work with GPS. If there is subsidence then how new are your maps from FEMA? Seems like the whole system of BFE's would need updating fairly often to be meaningful. I suppose the BFE's could remain static while the ground lowers and puts more and more buildings in the flood plain. ?ÿ?ÿ
NGS has suppressed the NGVD29 data in south LA due to subsidence. It's hard to find a control point with NAVD88 data.
?ÿ Probably not much help, but check this:
----------------------------------------------
AU3165* NAVD 88?ÿ ORTHO HEIGHT - **(meters) **(feet) NOT PUB
SUPERSEDED SURVEY CONTROL
AU3165 NAVD 88 (12/17/98) 0.7 (m) GEOID96 model used GPS OBS
AU3165 NGVD 29 0.94 (m) 3.1 (f) LEVELING 3
----------------------------------------------
AH6274* NAVD 88?ÿ ORTHO HEIGHT - 1.05 (meters) 3.4 (feet) GPS OBS
AH6274* NAVD 88?ÿ EPOCH - 2009.55
AH6274 **This station is located in a suspected subsidence area (see below).
AH6274 NAVD 88 orthometric height was determined with geoid model GEOID12A
AH6274 SUPERSEDED SURVEY CONTROL
AH6274 NAVD 88 (03/12/08) 1.12 (m) GEOID03 model used GP(2006.81)
AH6274 NAVD 88 (12/17/98) 1.3 (m) GEOID96 model used GPS OBS
-----------------------------------------------
AH6290* NAVD 88?ÿ ORTHO HEIGHT - **(meters) **(feet) NOT PUB
AH6290 SUPERSEDED SURVEY CONTROL
AH6290 NAVD 88 (03/12/08) 0.69 (m) GEOID03 model used GP(2006.81)
AH6290 NAVD 88 (12/17/98) 0.9 (m) GEOID96 model used GPS OBS
we are still using the 1985 FIRM maps in two parishes.?ÿ?ÿ
Then with bench marks in the area not reliable, the maps still referencing them, you are probably doing the only thing possible.
Does FEMA and the local flood managers agree with your procedure??ÿ
I would reference their endorsement, put it back on them somehow.
It's a difficult position to be in, a BFE but no direct way to reference it.?ÿ
I will contact the local flood managers.?ÿ ?ÿI was looking for the corpscon download just see the difference from the others.
