Vertical Datum

Scott Tikalsky · Post by **Scott Tikalsky** » Fri May 16, 2008 1:30 pm

OK... I need help in convincing a client to use NAVD88 instead of NGVD29 as the vertical datum for their project. I know that the Public Resources Code has language for using CCS83 for new State Plane coordinate projects. Is there some language, also, for using 88 over 29? I would think there is, but am unable to locate it.

Thanks,

D Ryan · Post by **D Ryan** » Fri May 16, 2008 2:00 pm

Look at 8850-8859 of the Public Resources Code. You might find the answer there.

bwatkins · Post by **bwatkins** » Fri May 16, 2008 2:23 pm

Scott you might want to also read 8860 & 8861 of the Public Resources Code before you point your client to this section.

dmi · Post by **dmi** » Fri May 16, 2008 7:20 pm

The short answer is NO. There is no requirement to use one datum over the other. Ther PRC basicsally states that if you use(calim) the protected terms then there are certain standrds that must be followed. The very best reason to use NAVD 88 over NGVD 29 is that NGVD 29 is a superceded datum. The infrastructure is no longer supported and therefore there are no recent observations that have been adjusted in relationship to a datum network . In areas of crustsal motion of any significance this means for all pratical puporse your NVGD 29 benchmark elevation is really just an asuumed elevation since it is not likely that you will be able to check into other evelations supposedly on the same datum.

Steve Martin · Post by **Steve Martin** » Sat May 17, 2008 10:45 am

How about the Federal Register notice making NAVD88 the official datum for Surveying and Mapping?

E_Page · Post by **E_Page** » Sat May 17, 2008 11:14 am

Whether vertical datum is important and which vertical datum to use depends upon the application of the mapping.

While FEMA maps are in the process of being updated, many communities are still on the NGVD29 datum. If that is the case for the area your project is in, and if flood zone certification might be a possible application of your mapping, you should stick with 29.

Answer your own question by determining the ultimate uses of your mapping efforts.

Scott Tikalsky · Post by **Scott Tikalsky** » Mon May 19, 2008 6:27 am

Thanks, everyone for your responses. I will review the PRC sections mentioned. Sreve, thanks for the Federal Register notice. That will also come in handy. We are involved in a design/build project along approximately 5 miles of the Pit River west of Burney. (RAM, you are not the only one that has to work in paradise!) The project owner brought "Sea Level Datum" into the area, back in the 30's, using barometric methods, accurate enough for them at the time. They then propagated the area using conventional methods. We have been working for the owner for a couple of years on other projects in the area, using NAVD88 as our primary vertical datum, then converting to the owners datum as necessary. For this design/build project, someone within the owner's ranks has decided that NGVD29 should be the datum. This is creating a lot of extra, unnecessary work converting to, in my opinion, an unusable datum. The info posted here should help me make a good case to do this project the right way.

Thanks again,