Do not silently ignore Walls shot data containing Instrument Height (IH) and Target Height (TH)

[Joe Walko]

Walls IH/TH data should be processed, not silently ignored. This is a defect, not just an enhancement, as Walls shot data is ignored with no warning to the user.

Many older compass and tape surveys were performed without a clinometer, instead the vertical offset of the "to station", relative to the "from station" was measured via tape, and records. Sometimes referred to as "flat tape surveys". A large Walls dataset I work with contains over 3,000 shots with IH/TH data.

See attached bundle of sample test data and related artifacts.

[Olly Betts]

Yeah, we really should at least complain here. The main reason we currently ignore this is because it looked rather fiddly to check and there were a lot of other Walls things to support which were actually used in the datasets people had provided me with. We can't just complain if we see tape=<something other than ss> because Walls defaults to tape=it. So we need to actually parse the heights and complain based on them and the current tape setting.

Since then I've added support for Walls diving surveys and I think that probably provides most of the needed machinery to properly check this. Thanks for the sample data - that'll help a lot.

Many older compass and tape surveys were performed without a clinometer, instead the vertical offset of the "to station", relative to the "from station" was measured via tape, and records. Sometimes referred to as "flat tape surveys". A large Walls dataset I work with contains over 3,000 shots with IH/TH data.

Is this the only way the taping stuff is used in practice (other than for diving data)? I'm wondering if the "Hybrid c&t shot" is a hypothetical example or something people actually use.

We might be able to simply map "flat tape surveys" to Survex's "cylpolar" style (similar to how we map TAPE=SS ORDER=AD and TAPE=SS ORDER=DA to the Survex's "diving" style).

[Olly Betts]

Just to check I have understood how these modes are meant to work, is this correct?

• If there's a clino reading, Survex currently mishandles unless instrument and target heights are the same

And depending on the tape setting:

• tape=SS: No other considerations
• tape=IT: Survex currently mishandles unless instrument and target heights are the same
• tape=IS: Survex currently mishandles if instrument height is non-zero
• tape=ST: Survex currently mishandles if target height is non-zero

If so, specifying instrument and/or target height when tape=SS and there's no clino should probably warn as those heights have no effect. Similarly for tape=IS, no clino and a target height; and tape=ST, no clino and an instrument height.

The "flat taping" use case I think I understand now (though how do you know you're actually taping level? A small angle off level changes the tape length very little, but any error in levelness is going to appear in the measured instrument vs target heights isn't it?)

Are IS and ST modes actually used? If so, do you have examples of usage?

[Joe Walko]

I'll try to provide responses to questions, and provide more info when I catch up with some backlog. In them meantime, to provide some context, attached is sample, one including hybrid angular inclination combined with IH/TH, from 1981, of a standard 'survey form' used by a cave research group in KY. The 'form' was a rubber ink stamped overlaid onto a 6"x4" spring-bound "memo book", a common format in the U.S.

Note the form has columns for "Vert Angle (°)" and "Vert Dist (FT)". Survey trips typically recorded either inclination in degrees OR vert distance, reflecting the height of the "to station" relative to the "from station". Previously I mentioned surveys which record only "vert dist" as "flat tape" surveys - a more common term, one I use, is "delta height surveys.

This particular survey book was from a survey training exercise. Four separate books from the class, with identical data, confirm the "hybrid" shots it contains were intentional, not recording blunders.

Maintaining original "stations" is a strong goal for resurveys in this cave system. In this example, it looks like recent collapse/breakdown obscures what previously were line-of-sight shots for X3->X4 and X4->X5. If I'm interpreting this correctly, the workaround, one which presumably was taught to be standard practice during this training, was to shoot to an alternate point 1/2 foot above X4, recording both the inclination and the 1/2 ft vertical offset, followed by inclination and 1/2 down from X4 to X5. P5->X1 involved similar shenanigans - note in sketch states "station is 1.5ft off the floor".

The hybrid shots in this survey thus appear a bit contrived - Why didn't they just add in new intermediate stations? Nevertheless, there is a scattering of such hybrid shots, ones not from training, in this large dataset. They are rare, but exist. While mostly confined to old surveys, one well known cartographer continues to occasionally use these hybrid shots (much to my chagrin - this trip an alert when I run my 'screening' script).

(sample survey book image attached)

[Joe Walko]

samples images from survey book with hybrid shots and "vert dist" columns