NAVD29 adjustment to NAVD78

[TC54]

Does anyone know of the method or a reference document which explains how to adjust an elevation from one vertical datum to another?

Specifically, I'd like to know how to adjust an elevation from NAVD29 to NAVD78 in the area of Toronto, Canada.

Thanks

 

[KRSServices]

Your best bet is to get rid of the old information sheets (NAD 28) and get updated NAD84. Here in Alberta, Land Titles has a website where monument information can be procurred for free. This ensures that the information regarding the monument is current and up to date.

In some cases the adjustment factor may have changed and the conversion may not be too accurate or purposes of tight coordinates.

My overall suggestion is to toss the old cards and replace them with newer ones.

KRS Services

http://www.krs-services.com

 

[TC54]

Thanks for the response. Unfortunately, I don't think it solves my problem.

To clarify my problem, I need to check to see if another consultant's design for a residential development will jive with the grading of my road design (i.e the design elevations on the property line between the road and development needs to be the same). However, the development design was completed using a different datum (NAVD29 - the standard used by the jurisdiction responsible for the development). In order for me to check these elevations, I need to change them to the datum our client uses (NAVD78 used by the county). The surveyor for the development suggested we lower the NAVD29 elevation by 14cm to make the adjustment. I just wanted to verify if this sounds reasonable.

As this is just for pregrading of the development (the road will be built a year later), the values do not have to be 100% accurate, but I want to know if the elevations are in the right ballpark...

 

[KRSServices]

I ubderstand your problem, but I am not sure as to why you think you need to correlate the two design elevations (real world) to the datum? If the same monument is used, regardless of whether NAD28 or NAD84 is used, the elevation on the card is correct. It the design elevations were referenced off this monument, that information must be assumed as correct.

If the marker was re-established, then, again, the new elevation is to be assumed as correct. As a point of record, you may want to determine the disturbance difference.

I have had to convert designs that for reason unknown, the engineer referenced all datum to the plane as opposed to real ground. It was a nightmare to correct and adjust everything to real ground, particularly for the coordinates.

To assist you, I suggest the following:

1) Get your surveyor to tie-in the monument to the boudary of your project and the development. The purpose will be to provide the real-ground co-ordinates to the monument and some common tie references. The elevations will also recorded by the surveyor.

2) Compare the (real ground) elevations of the consultant's drawings, paying particular attention to the elevations of the monuments. They should match. If they do not, the monument may have been disturbed and therefore the most recent elevation (new datum) will have to be used. This elevation does not change with respect to the plane (use of factors), only the co-ordinates change, therefore the datum adjustment on the drawing will only affect the marker, not the design because all other elevations (relative to grades and the sites) were relative to the datum elevation, and since they have not changed, the information will still be valid.

3) Call me if you need further explanation or assistance.

KRS Services

http://www.krs-services.com