Datum - Hawaii 1

[vicki932]

I was hoping someone may be able to help me understand elevations that I have from an area on the island of Kauai in Hawaii. I have elevation information from 1935 with no datum indicated. The same location using USGS info indicates a difference of 8 to 10 feet less than the plans dated in 1935. I'm assuming that back in 1935 mean sea level was used as the datum, but that still doesn't seem right.

I'd appreciate any help you may have to offer on this.

Thanks

 

[francesca]

I'd start with the assumption that it's NAD 27 and try Corpscon to convert them.

http://crunch.tec.army.mil/software/corpscon/corpscon.html

If by USGS info you mean a regular topo map, I wouldn't put much store in those elevations. They're accurate to 10 ft at best.

 

[vicki932]

Thanks for the response. I'm mainly trying to figure out the difference in the DEM data of today (1' contours), which is about 8 to 10 ft off from elevations on plans from 1935 at the exact same location. Since no datum was given, I'm a bit lost as to what someone else might do. I'm not sure what datum the islands of Hawaii used 80 years ago (vertical datum).

I appreciate your response.

 

[francesca]

It seems you don't understand my response because NAD 27 is the North American (vertical) Datum of 1927, and a likely vertical datum for your 1935 survey. What is the vertical datum of your "DEM?" Any CAD/GIS Digital Elevation Model (DEM) can be drawn on any contour interval, including USGS Topo/Google Earth topo, which are grossly inaccurate on a small scale. The Corpscon software provides a conversion factor from NAD 27 to NAD 83 (a likely vertical datum for your recent survey, especially if the horizontal datum is State Plane).

 

[vicki932]

Thank you. You're right, I didn't understand, but understand now. Thanks.

 

[msquared48]

What you both don't understand is that Kauai is gradually sinking back into the ocean... Could this be a clue?

Hope this helps...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[vicki932]

It could be, but the problem is that the data is showing that it's rising up and not down.

................

 

[BlastResistant]

All the Hawaiian islands are sinking, and sea levels are rising. If the current level is less than the 1935 data with respect to MSL, doesn't the current data indicate that it is sinking? We are losing anywhere from 4 to 12 inches of shoreline per year (depending on the location). About a third of that is attributed to rising sea levels and the remainder due to sinking. We have lost approximately 25% of beach square footage since 1950. The islands are also drifting horizontally to the NW at a rate of about 9cm per year. Not sure exactly what you are trying to accomplish, but if horizontal and/or vertical data is critical, you should really have a local Hawaii based land surveyor re-survey the area in question if your data is older than a year.