[time-nuts] TBolt self survey

Thu Feb 11 15:30:09 UTC 2010

Thanks for all the info Arnold. I will study all these links ASAP.

I had a Garmin 12A (many years ago) that gave the actual height close to the survey value of 928M ASL. I wonder if those calculated the real values..

>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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-- 
Raj, VU2ZAP
Bangalore, India. 