5.75' in SouthCentral Alaska
Just because I'm paranoid, doesn't mean they aren't out to get me.
Simple. I basically tell people that 29 is an antiquated plane based on sea level that took years to develop, so that elevations could be compared across large areas. Almost as if to prove once again that the world isn't flat. But this datum puts everything on the same page so they can compare, and help define drainage basins.
It took them 60 yrs and a bunch of satellites to really figure it out, so they based a new datum on more real numbers that factor in a lot more things. The difference in our area is around xxx (here it's about +2.8') to convert. In some areas of the US it's -1'. They are usually ok with that explanation.
At the end of the day, they really want to know how it impacts them. It's our job to make sure we get it right, thanks to VERTCON
I don't hate the Colonel, I just don't like his chicken. I like Popeye's spicy better.....
Thats no help...but thats Funnnnnnny!!!:-) 🙂 :clap:
That I can use!!! Thanks.
How do you... 29 flawed
> Because neither feet nor meters have an inherent flaw...
I suppose if you let the foot stand on it's own; then it has it's merit. It's when you mix in the inch, you run into trouble...
And what about the conversion to meters; tell me that doesn't screw with a few peoples heads.
> But 29 has more flaw than the 88 flaw...
The 88 has flaw? I was told it would take over the world....
> Try to draw the flaw in the geodetic coleslaw... yeehaw...
Nature is perfecly flawed; you can define it any way you like and you won't be wrong...
Psssst...The Walton family (WalMart) took over Sanders' seat...
0.1foot in central Arkansas...FEMA Appendix B calls insignificant...Arkansas=WalMart...Just saying'
😉 DDSM
In 1929 the federal government by some agency used 100k kilometers of minimally adjusted leveling through 26 tidal stations to create the Mean Sea Level datum of 1929. Because they could not adjust the data and there were errors, they held each tide station for each area.
In 1973 the federal government renamed MSL 29 to NGVD 29 to reduce the confusion that arose and apparently a number of engineering mishaps due to some engineer unwittingly determining MSL was actually local Mean Sea Level.
In 1988 the federal government by the national geodetic survey compiled over 600k kilometers of leveling into a least squares network. They were able to average the elevations of more than 26 tidal stations to find the average sea level. They then chose the tidal station at Father's Point, Rimouski, Quebec, Canada as the basis for NAVD 88. This station was very close to the average and is the basis for the Great Lakes Datum of 1985.
Simple answer:
Like most Americans, the Earth is wider in the middle and flatter on the top. Sea level is not the most stable basis for a zero, but it is the best we have been able to rely upon. Now we have better measurements on an ever changing sea level. They will come out with a new datum someday and like NAVD 88, we will avoid using it until absolutely necessary.
STUDY YOUR FIS
Joe,
Could you tell me about VERTCON and the actual difference between NGVD and NAVD in your neck of the woods?
1. VERTCON was not intended to be relied upon completely by surveyors. The documentation clearly states that it is not adequate for 1st, 2nd or 3rd order heights.
2. FEMA does not know this stuff. The FIS in Barnstable county lists the difference between NGVD and NAVD to be 0.9' when in fact the difference is about 1.06' (on average). The greatest difference is about 1.3' in Orleans, MA.
Why do I ask? In the case of someone in Barnstable County, someone using an old elevation certificate will be better by 0.16' (on average) than someone who orders a new survey based on true NAVD. Let NFIP make the conversion...
Thadd
About 5 feet where I'm at.
Last year I was doing some topo for dam reconstruction site design. Most had been done 20 years ago from aerial. The change was so close to 5 feet and the topo had 5 foot contours so I told the engineers just to relabel the contours. At first they thought I was nuts but after we went over it they agreed. They did put a big note on the plans sent to state dam safety explaining the datum change. Sailed right through.
STUDY YOUR FIS
I don't use Vertcon very much, if at all.
Most of the CDOT monuments have both the 29 and 88 datums on them.
But the difference around here is about 1.08'...enough to drive you nuts.
How do you... 29 flawed
> What is wrong with inches, varas, rods, and cubits...
I prefer smoots; but that's just me..;-)
> isn't decimal feet just junior varisty metric? 😉
I like it! :good:
If you think the confusion between NGVD 29 and NAVD 88 is hard to explain and/or account for just wait until NGS releases the new geopotential datum to replace NAVD 88 in 2022. Depending on where you are in the country the height differences will be in the range of near 0 (southern Florida) to more than 1 m (all of WA, parts of ID, OR and MT).
STUDY YOUR FIS
Case in point. In my county, the cities and county all have extensive runs of published bench marks, all on 29 datum. ADOT has a few but they typically are either gone or virtually inaccessible. The NGS stuff is also on top of mountains.
Now that FEMA requires 88 the options are to run OPUS or use VERTCON to adjust. Most of the elev certs I do are in rural areas, with a county BM nearby.
The 2 hr OPUS solution is not cost effective, but hitting a couple nearby 29 BM's is. Just note the conversion factor on the EC and it seems to keep people happy.
> If you think the confusion between NGVD 29 and NAVD 88 is hard to explain and/or account for just wait until NGS releases the new geopotential datum to replace NAVD 88 in 2022. Depending on where you are in the country the height differences will be in the range of near 0 (southern Florida) to more than 1 m (all of WA, parts of ID, OR and MT).
This is depressing even if we know it is coming. Hopefully I'll be long retired by then so I can sit back and watch
Now that's funny. It looks like they will be back almost exactly to the NAVD29 numbers in my area. NAVD29+2.3'=NGVD88-2.4'=the new value. :-O
Global warming...
I live and work at an elevation around 7000' feet. Had a client call me a few months ago to tell me that his lender rejected our El. Cert. because "the elevations are wrong". After some digging I was able to contact the reviewer directly and asked her what the problem was. "You have too many numbers in your elevations" was her reply. "What?" I asked. "You have too many numbers in your elevations" she repeated. I told her I didn't understand what she meant and asked if she was referring to the single digit to the right of the decimal point. She struggled with that concept for a bit and repeated "You just have too many numbers in your elevations". "Your numbers should look like 100.2" "You have 7025.2". You should only have three numbers there and you have four". I said "Well, that is the correct elevation of the site. We are located about 7000' above sea level and are reporting the elevation of the property." She thought for a bit and said "Well, I guess that will be all right then".
I get that question all the time. I try to keep it simple and not use a lot of survey jargon to get the point across, like "in 1988 the National Geodetic Survey recalculated the elevation of all the known elevation points across the county. In this part of Florida, the new elevations came out about 1' lower than the previous elevations... The new flood maps were adjusted to be based on the 1988 datum, so we have the remeasure the elevation of your house on the 1988 datum."