TopoDroid & GEO-POINTS

[Apocalypso]

Hi,

Here I am once again

I'm playing with cave geo referencing of one old paper based survey and I'm not sure how to find those two values:

Any help is more than welcome...

 

[andrew]

I'm guessing here, so it may be useful or it may not. I have hardly used topidroid and not this feature. So you have been warned

I'm guessing that one is the geoid height and the other the ellipsoid height, which is probably height above see level and height as given by a none corrected GPS in wsg84 mode, respectively. I suspect only one is needed. They can vary drastically, however most GPS now give corrected altitude. If anyone wants a more detailed explanation see

1-Mean Sea Level, GPS, and the Geoid

Esri is the world leader in GIS (geographic information system) modeling and mapping software and technology. This site features GIS mapping software, desktop GIS, server GIS, developer GIS, mobile GIS, GIS Web services, business GIS, Internet mapping, GIS solutions, GIS training and education...

www.esri.com

 

[Ian Ball]

In Lev Bishops 2003 article entitled "GPS Co-ordinate Transformations: an Overview" in Compass Point, the journal of the BCRA surveying group there is this on the second page.

Heights
Height datums are traditionally separate to horizontal datums for practical surveying reasons A distinction needs to be made between orthometric heights (heights relative to a geoid) and ellipsoid heights (heights relative to an ellipsoidal approximation to a geoid).
British OS uses orthometric heights relative to ODN (Ordnance Datum Newlyn).
GPS is relative to WGS-84 ellipsoid. Since geoids are irregular in shape, the geoid separation is not constant over wide areas. This makes conversion from local to global height references more difficult than the horizontal.

I am no surveyor and may well be wildly off the mark (surveying pun?) but it would suggest to me that you need to specify a height for your reference point as either a orthometric or ellipsoidal value and the programme processes the survey differently based on which datum you are using.

(All well explained in the link above from andrew)

 

[Apocalypso]

HI,

I'm back first of all 10X 2 both of U

I made couple of tests in mean time and no matter what I put in any oh those two fields I'm always getting the same result when georeferencing sketch so I would say it is not crucial information but....

It is always better to have right info about altitude here and it is definitely enough to input only one value. From now on it will be H geoid which is basically altitude value that I'll get from my Garmin handheld.

10X once again

Teo