[time-nuts] TBolt self survey

Arnold Tibus Arnold.Tibus at gmx.de
Thu Feb 11 13:45:59 UTC 2010


Raj,

don't trust the height asl from (older) maps. There are so many different local 
sea level definitions around the world, which are not yet WGS84/ EGM96 
conform (WGS 1972 would be good as well)! 
I had similar problems comparing swiss and and german map values 
with the Shuttle Radar Topography Mission (SRTM 2000) values.

On http://www.colorado.edu/geography/gcraft/notes/datum/edlist.html 
I found interesting informations about the differences (tables).
Eg. India used the Everest ellipsoid (India 1956, for Inda and Nepal) 
where the differences to WGS84 are (d = delta): 
dX= 295, dY= 736, dZ= 257 meters, what might explain already some 
discrepancies. Attention to the horizontal displacements as well!

So nothing to worry, as already correctly explained by several other 
fellow Time Nuts, use the elevation found with the auto position function
in LH, this is the right GPS value for this purpose ( perhaps moving around 
up to 1 m or similar), which is of no concern I think. Mark Sims and 
John Miles did really a good job! 
(The self survey function in Thunderbolt Monitor does give a similar 
result with somewhat less resolution/ precision.)

Searching around I found another page with good informations for 
beginners: 'Datum of a GPS Receiver'
http://homepages.slingshot.co.nz/~geoff36/datum.htm

The therein mentioned calculator 
'Geoid Height Calculator NGA EGM96', url:
http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/intpt.html
does give the same results...

enjoy the time

Arnold, DK2WT

On Thu, 11 Feb 2010 06:26:42 +0530, Raj wrote:

>My GPS reading is about 850 +- 1
>The "official" height ASL = 920

>This calculator gives me ~936

>So what should I use ?

>Thanks
>Raj

>At 10-02-10, you wrote:
>>Try to find your correct height here:
>>http://sps.unavco.org/geoid/
>>
>>If you put in your correct coordinates without any input for the elevation, 
>>you will get theorthometric height, which is the GPS ellipsoidal height 
>>minus geoid height. That is the amount you need to subtract from 
>>your Thunderbolt GPS height in order to get your local geodetic height 
>>(WGS84). 
>>
>>Filling in the Thunderbolt GPS height you will get the 'real' height 
>>for your antenna...
>>
>>Some explanations:
>>http://www.unavco.org/edu_outreach/tutorial/geoidcorr.html
>>
>>On my location I have always to reduce the GPS height by 46.623 m.
>>Good luck with TB and the Lady Heather,
>>
>>Arnold, DK2WT
>>
>>
>>
>>On Wed, 10 Feb 2010 14:20:36 +0100, EB4APL wrote:
>>
>>>Raj,
>>
>>>Ask them about ellipsoidal and orthometric heights, or the local 
>>>difference between the geoid and the WGS84 ellipsoid.  Also your long 
>>>and lat may change depending of the Geodetic Reference System chosen.  
>>>By default GPS receivers use WGS84 and ellipsoidal height.
>>
>>>73 de Ignacio, EB4APL
>>
>>>--------------------------------------------------------------------------------------
>>>Raj wrote:
>>>> Thanks Stan,
>>>>
>>>> That worked, I did not see the  control menu properly!
>>>>
>>>> I set the co-ords to 0 0 0 and started survey. The Tbolt found the Lat long withing a few minutes and only the altitude is about 70 M below the official figure for my place. I live a a few hundred meters from Bangalore center and I am at the same height.. must check with the survey dept. down the road!!
>>>>
>>>> 73
>>>> Raj vu2zap
>>>>
>>>>
>>>> At 10-02-10, you wrote:
>>>>   
>>>>> Hello Raj,
>>>>>
>>>>> using the trimble T'boltMON software:
>>>>>
>>>>> Start with a "factory reset" in the control menu
>>>>>
>>>>> follow previous guidance of enabling the "save position" capability,
>>>>>
>>>>> Then to restart a survey
>>>>>
>>>>> go to the control menu (on left),
>>>>> select "restart self survey"
>>>>>
>>>>>
>>>>> In lady Heather V3.0 beta of 21 Jan '10 ( available from the KE5FX website):
>>>>> keyboard enter a "s", for survey
>>>>> then "s" for a standard survey,
>>>>> use the default of 2000 samples,  for a start
>>>>> Then "enter"
>>>>>
>>>>> Time for me to go to bed.
>>>>>
>>>>> Please advise of progress.
>>>>>
>>>>> Stan,  W1LE
>>>>>
>>>>>
>>>>> Raj wrote:
>>>>>     
>>>>>> Thanks Stan,
>>>>>>
>>>>>> I did that but I can't where to "start the survey" in TBolt. In lady heather there
>>>>>> is a key for it.
>>>>>>
>>>>>> At 09-02-10, you wrote:
>>>>>>  
>>>>>>       
>>>>>>> Hello Raj,
>>>>>>>
>>>>>>> that location is probably where the coordinates were last saved. or there abouts....
>>>>>>> Or those are the default factory settings from memory,
>>>>>>> and new coordinates were never saved.
>>>>>>>
>>>>>>> redo the survey and save the results.
>>>>>>>
>>>>>>> using T'boltMON V2.60 on a PC:
>>>>>>>
>>>>>>> go to menu "set up",  then "self survey",   click on the "save position flag"
>>>>>>> then "set survey",
>>>>>>> then "save segment",
>>>>>>> then "close"
>>>>>>>
>>>>>>> redo the survey and afterwards verify the local coordinates were saved.
>>>>>>>
>>>>>>> Lady Heather (software) can also be utilzed for these functions.
>>>>>>>
>>>>>>> Any problems, please advise.
>>>>>>>
>>>>>>> Stan, W1LE
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Raj wrote:
>>>>>>>    
>>>>>>>         
>>>>>>>>       Can someone point me to info that would clarify why my Tbolt's self survey puts my house (AFAIK ~920M ASL, 13N 77.35E) in the Pacific ocean, south of Vancouver and west of Seattle and about 10 meters under water. I am doing something wrong for sure!
>>>>>>>>
>>>>>>>>       My sons iPhone gives me right co-ordinates but it's way off the altitude.
>>>>>>>>       
>>>>>>>> Regards
>>>>>>>>           
>>
>>
>>
>>
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>-- 
>Raj, VU2ZAP
>Bangalore, India. 







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