Hi folks,
I had a small attached terrace house to survey today with no access down the sides (buildings abutting on both sides). The only access to the (tiny) rear yard was through the house itself, which involved traversing through a glass window (TS distance and bearing). I shot a check point inside the house itself also, (not through glass), then took a shot back to this once established in the back yard (through glass), which checked to 4mm (good). setting up inside the house on the polished floor was avoided as we didn't have any decent way of protecting the floor and preventing tripod slip.?ÿ
However the fencing and occupation located at the back seems a bit out - maybe just the way it is, but I'm wondering if anyone has done this sort of thing more often than I have and found any significant errors (with incorrect bearings and/or distances through glass). I may also have a backsight issue, but this can be checked.?ÿ
Cheers
I avoid shooting through glass.?ÿ No telling how much refraction and distortion is created, especially if pointing through at an oblique angle and not perpendicular to the glass.
Your distance would be affected by no more than the thickness of the glass. But your line of sight will be refracted - literally like a stick in water. It's the angle you read that will be most affected, not the distance.
My memory is a little fuzzy on this, but I recall having distance deltas more on the magnitude of a couple tenths the last time I shot through glass.
It would be easy to replicate. Setup in your yard and shoot a prism thru a window. Then raise the window and shoot it again.
I would be more concerned about the line.
Shooting thru glass will not always produce the same results as most glass contains flaws that can deflect the view and infrared or laser beam.
It will usually give close results and that may or may not be within prescribed standards.
I only do it when there is no other way to locate something.
I've done many practical field tests. Angle is most affected, distance is also affected. Glass coating and material composition have an effect. One window will not necessarily have the same effect as another window made of different composition of materials. Some glass when shot through perpendicularly did not have a material effect, other glass did. Again both distance and angle will be affected and I determined for myself only, that I could not rely on any shot through glass without adequate check shots to prove my resulting location. A high refractive index will produce a larger discrepancy than a low refractive index.
Back in the 80's I was hired to observe a first order Astro azimuth and then traverse into a building carrying the azimuth to calibrate inertial navigation units for fighter jets. We had to go through a wall but for some reason it could not be open to the air (I don't remember why). They had a special very thin expensive membrane that we shot through that supposedly did not distort the light Ray. We also had to hit two optically flat mirrors on the way in to get around corners. Total traverse length (no distances, just angles with a DKM3A) was a few tens of feet
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@john-hamilton I remember seeing similar work in the early 1980s at Draper Laboratory (the fountainhead of inertial navigation) in Cambridge, MA.
In all my years, I have never shot through glass.
Bad practice.
Not good practice, but occasionally there isn't a better option, either due to the internal arrangements or the fact that the windows can't be opened for some reason.
You can minimise errors (and get a good guide to what they are) if there are two windows to observe through. I print targets on a lightweight paper and fix one to the inside of each of the windows. Fix them from outside (either by intersection or non-contact, or better by both methods). Then traverse between them inside. If you did the outside fix by non-contact remember to add the glass thickness to each shot.
If you can get a room with three windows a decent resection is possible - probably more accurate than traversing up and down the stairs for the upper floors.
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Thanks for the responses chaps.
clearly I didn??t do it by choice, and in the perfect world you would not do it, but sometimes the needs dictate the ends. Understanding the possible errors is the key, I think, rather than just having a ??never ever until hell freezes over? attitude. The errors I am looking for are far in excess of those mentioned, so I am thinking a site revisit to check the angle in. Gonna have to make up an excuse to inconvenience the tenants again - I??ll come with free coffees and a ??I just gotta check some things? excuse.
