Help With NGVD 29 ?

You could find the superceded 29 elevation at the NGS site somewhere and start from there.

I've Never Done That

Paul in PA

I thought that NGS is only putting navd 88 on the recovery forms now. You can always use corpscon or vertcon to find the elevation difference and edit the datum. not sure how OPUS would handle it though

Assuming I am reading your question correctly, a couple of thoughts:

- Look on some old FEMA maps that you or someone else may have laying around. Perhaps it was a reference mark on the old maps.

- Ask around of local surveyors to see if anyone has the old data sheets laying around. I know I have hundreds of the old sheets from as long as 40 years ago laying around the office.

- Check with the municipality to see if they have any old plan sets laying around for projects in the area that may have shown that as a project BM.

- Perhaps the COE may have some information if they've ever done anything on the river.

- Or some of the old SCS studies may have the BM shown in their detailed studies.

- Haven't looked for them, but I've been told USGS has all their old sheets on line. When their level lines crossed paths with other level lines they generally included other BM's in their data.

Lots of possibilities for finding old BM info.

Couldn't you just do the GPS run with the Opus derived verticals (suggest hitting a couple of extra BM's) and then do a vertical translation post process. The elevation shift shouldn't be enough to mess up your geodetic calcs....and you have checks against the other BM's for accuracy and redundancy.

This Monument Never Had An NGVD 29 Elevation

New Jersey Geodetic Survey monument established in 1993, so there is no superceded value to access. "KV7129"

Within sight of this is the GPSable location of an NGVD 29 monument that has been made inaccesible by the homeowner.

For KV7129:
ELLIP HEIGHT 129.123 m
GEOID HEIGHT -33.22 m GEOID09
NAVD 88 162.3 m

Before using VERTCON is the published data suitable?

OPUS-RS
EL HGT: 129.151 m 0.028 m
ORTHO HGT: 162.350 m 0.037 m (I requested GEOID03)

Is VERTCON particular about the GEOID model?

Paul in PA

As I understand the use of vertcon or corpscon ( I use corpscon6) is that it's a direct conversion based on geographical location.

Since you have the NAD 88 info a conversion using corpscon should be easy enough. Check with your NGS state coordinator for more particular instruction.

I have found about 0.1' difference between a direct level run tie and the corpscon conversion. I have not noticed any difference between geoids, at least not significant enough for my work.

Dtp

The best way to determine the difference is to run a level between a point in the desired datum and the point you occupied. The next best is to occupy a nearby point that has NGVD29 values.

There are three points in the NGS database within a mile of your control point. Two were not found and the other, KV1250, is a mile away and is 130 feet different in elevation. Comparing the three points shows an approximate difference of 21 centimeters (0.211, 0.213, 0.213) between NAVD88 and NGVD29. I would get a short static session occupying your control point and KV1250 and vertically constrain your adjustment to KV1250.

What Rob Said.:good:

This Monument Never Had An NGVD 29 Elevation

Howdy,

Hybrid geoid models, named GEOID##, are used for NAD 83 values; gravimetric models, like USGG ##, are not.

First of all, be aware of the possible impact of new CORS station coordinates on heights via OPUS. On this issue, the following answer is provided on the OPUS page:

"A: More or less. The GEOID09 model was built from CORS96-derived ellipsoid heights, therefore orthometric heights obtained will differ by at least the change in ellipsoid height. If orthmetric heights are critical to your needs, we recommend that you submit a solution through both submission buttons and evaluate the differences. Shortly after the completion of the passive adjustment a new geoid will be created using the consistent set of new ellipsoid coordinates from both CORS and passive control."

The height reported as NAVD 88 on an OPUS solution uses the computed ellipsoid height in NAD 83 (h) in combination with Geoid model (currently Geoid 09) value (N) for the geoid-ellipsoid separation at that point to derive the NAVD 88 value (H). The relationship is h - H - N = 0 (+/- errors in h,H,N).

Given a height in NAVD 88 and no other observations from it to NGVD 29 network points, we can derive an NGVD 29 height by either VERTCON, polynomial fitting or even merely taking an average shift in the area and applying it to the point.

If ties to NGVD 29 points are not feasible, I suggest VERTCON (see http://www.ngs.noaa.gov/TOOLS/Professional_Surveyor_Articles/VERTCON.pdf

VERTCON (CORPSCON uses VERTCON) is the most accurate of the three methods. The model's official accuracy is 2-cm at one sigma. Even so, users should take care to insure that there are not localized problems with the model. Compute values for points surrounding the project area to insure that no large differences in shift values exist in the area.

The tool interpolates values for a point of interest using a grid of differences between points in both NGVD 29 and NAVD 88. These differences also incorporate modeling to correct distortions in NGVD 29. Since the positions for bench marks are usually not well determined (mostly scaled from topo maps) inputs can be either NAD 27 or NAD 83.

Hope this helps,

DMM