Posted by J.Bradley on March 08, 2008 at 17:00:59:
In Reply to: Re: Use of Theodolite Striding Level
Hi Gary,
A striding level is not strictly required.This is particularly so if you have a coincidence bubble on the vertical circle(as you do with the DKM-2) better still is the DKM-2A-which has a liquid compensator.Level the instrument as accurately as you can with the alidade(horizontal)bubble, then use the split bubble on the vertical to fine level by taking half the error out with the Tilting (Clip)screw and half out with the foot screws in the 0,90,180 & 270 horizontal positions. This will precisely level the trunnion axis which is the function of the striding level - however the instrument must retain this fine levelling during your observations!! Alternatively it is possible to read the dislevellment on the vertical circle by taking the vertical angle readings at 90 degrees left and right of the Line-of-Sight & computing the correction. This is the most accurate method as the axis must only remain stable for these few readings. The DKM-2A can read to 0.3 arc seconds
RMSE which is considerably more accurate than any striding level. A special version was manufactured by Kern, the DKM-2AEM - the M standing for micrometer which was mounted to the side of the VC (Vert circle) (E- was erect image of the telescope). The reading was subsequently applied to the horizontal angle as a correction for steep sighting. This correction is only significant when the targets you are sighting are at very steep angles (Sun Shots etc). I have depressed the telescope of a Geodimeter by about 30 degrees downwards and set 0 on the horizontal circle, then by keeping the instrument locked horizontally and elevating the telescope by about 30 degrees above the horizon (that is a vertical angle changes by about 60 deg total, the horizontal circle can change by 2 minutes of arc or more because of trunnion axis dislevellment and the instrument has a dual axis compensator. An ideal situation as the correction is measured and automatically computed and applied by the mi
croprocessor to the horizontal direction. So if you are look!
ing fo
econds accuracy for steep sighting then the correction is somewhat important. With a dual-axis instrument the striding level is definitely redundant! The best book on the subject of instrumental errors is "Surveying Instruments" by Fritz Deumlich. The English out-of-print version is available from ABE Books. If you send me your email I will forward a more detailed description from Wild Heerbrugg (Leica)of this particular axis correction.
Regards
John