During the preparation of an earlier paper (p/001/10) there was some question as to whether the radial-arm inclinometry of July 1997 reflected the track shape directly, or whether perhaps the results were being affected by some sort of cross-coupling. Suspicion focussed on an "inverted hump" feature, the suggestion being that results for wheel three were being polluted by effects on wheel 1 being transmitted through the structure. The conclusion is that the inverted hump is a real feature and the radial arm inclinometry agrees well with the Leica inner track data.

Data used

The inclinometry data was the July 1997 set, 970712RADY. The Leica data were taken from IP's file "inner.wk4". Note that the Lieca data were taken before az-track welding and the inclinometry was done afterwards and about 2 years later, so we expect some discrepancies to be present.

Results

A plot was made (figure 1) with the Leica data and the inclinometry results. In order to align the two I transferred both to a common azimuth angle as follows. I generated a list of azimuths from 0 to 360 in 0.25 degree intervals. For each azimuth I added an offset (see below) then looked up in the original data the result for the next-highest azimuth. The offsets were:

Lieca inner and outer: -72.3 degrees
970712RADY: 0 degrees.

The azimuth shift fpr the Leica data was derived as follows. In IP's presentation notes dated 27 June 95 he features the Leica plot captioned "Azimuth from wheel 1 on 1/2 = 252.5 degrees". The joints are also labelled on that plot. I infer that if we shift the data by 252.3 degrees we will see the results as if for wheel 1 in the telescope azimuth system. Shifting a further 180 degrees we should see wheel 3, and so a correlation with the radial-arm result.

Commentary

Looking first at the "inner" Leica result and the RADY result (middle and lower traces in the figure): I see a fair agreement in terms of peaks and troughs, especially the "inverted hump" feature at around az = 50 degrees. There is some disagreement in the azimuthal location of the "spikes", but I understand from memos written at the time that the Leica data was a little uncertain in azimuth.

The "outer" data are harder to understand, but I gather from discussions with IP that the "outer" data may have been taken at radius beyond the part of the track swept by the wheels, and so for now I am discounting that data.

Conclusions

I conclude that the radial arm inclinometry is giving us, in this case at least, a good picture of the profile of the track as wheel 3 sees it.