• Hello From Descent

    The publication date for issue 289 is the 10th of December, meaning subscribers should receive their copies during the week leading up to that date. It is also available from caving suppliers such as Inglesport and Starless River, or from our new website

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TopoDroid & GEO-POINTS

Apocalypso

New member
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:
Screenshot_2022-08-14-11-37-56-39_2e9ad78c23f43fb331f615e746f768a5.jpg


Any help is more than welcome...
 

andrew

Member
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

 

Ian Ball

Well-known member
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

New member
HI,

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

download.jpg


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....

033.jpg
034.jpg


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
 
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