Using a 2" Zoom90 mid morning and on 250m backsite I was getting a bigger range of horizontal angle differences than the angular spec and the prism pointing error suggest. 2" over 250m + a gmp101 should be 3-4mm but I was seeing them jump around 5-20mm.?ÿ Distances stayed constant through out. Instrument and target stayed on level throughout as checked them at the end. Was fine on <100m backsight mid afternoon.
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There were no obstructions on line as I checked through telescope when I tried taking some manual OBS to check (I was getting around 5-10mm angular differences) but the backsight wasn't as crisp as if it was only 50m away so must have been some light heat shimmer (only about 14 ?øC so not super hot) but still could make out backsight fine, it wasn't a fuzzy blob.
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Does the light heat shimmer really affect pointing that much?
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Also with observations spread over 2hrs what sort of drift would you expect to see when doing final check to backsight (assuming instrument and target stay on level)?
Heat shimmer affects readings quite a bit.?ÿ If higher accuracy is required?ÿ I?ÿ usually take an average of 5 or more shots in each face. This is easily done in Carlson's SurvCE(PC) where you can set the number of readings to average in each face for both the backsight and the foresight.?ÿ The instrument's accuracy also affects the readings. With Leica TPS or TS robots and the Zoom 90 series that use?ÿ the ATR to locate the target, I found that using a smaller prism like Leica mini helps. Because of the the heat the target looks like it is oscillating ,?ÿ so I?ÿ assume the ATR picks a random moment to set the angle. The smaller the reflective surface of the prism?ÿ the smaller the oscillation. I did some testing on a hot day and?ÿ the best accuracy I got was in this order: Leica mini, Leica 360mini, Leica 360,?ÿ Leica Circular prism. We?ÿ have both a 2" and a 1" Zoom 90 robots?ÿ that perform very well.?ÿ I take backsight readings at 250-300m all the time in the summer, but the foresight never exceeds 150m.?ÿ Best time to work in the summer is between 6 and 7am. That's when I do my control points.
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Thanks for that input, the smaller prism makes sense and I do usually use the gmp101 for all my pole work (even topo).
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When you did the prism testing did you get similar tracking behaviour, i.e. bearings jump around but distance is consistent?
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Do you recall the magnitude of the typical variations in the individual measurements in the averaged shots you take?
The testing I did was actually staking out a point about 150m away from the robot in strong sunlight at noontime with a temperature around 30 Celsius. I set each prism on the pole with a bipod to keep it steady and I took several readings of the same point noticing the "jumps" in the left-right direction. The In-out was almost constantly the same number proving the fact that the distance measurement wasn't affected by the heat shimmer. The angle reading, on the other hand, was definitely affected. The left-right readings varied from 2-6mm with the smaller prisms to 8-12mm with the Leica Circle. I did the same test in the winter on an overcast day and a few degrees below freezing and the "jumps" disappeared. All the prisms performed the same. I extended the stakeout distance all the way to 230m away from the instrument and I still did not see any variations between readings with the Leica Circle and the 1" Zoom 90. Manually sighting the center of the "oscillating" prism should yield better results. I believe the ATR does not map the whole oscillation to determine its center, instead, it locks on a momentary virtual position of the prism, then it measures the distance and because it is using a wide infrared beam, it will hit the reflector anyway. Averaging multiple readings should be used when weather conditions are less than ideal.
Thanks for all those details! Reassuring that what I was seeing is not unusual.
In my experience, on hot days the angular error with an instrument increases with distance 'up to a point', and then you start seeing much larger variations, particularly in vertical, as the visibility and shimmer picks up. I used to do a monthly monitoring run with shots up to 400m. For that job I was usually reading the weather forecasts a week ahead of the job, picking the best days, and often starting the job at the crack of dawn, just to avoid heat shimmer, wind, etc later in the day.
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Crikey, at 400m you'd hardly be able to see a prism with Aussie temps during the day!
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I'm assuming robotic total stations work still in dark given their technology, wonder if any monitoring gets done at night in summer so temps are consistent and low?
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I've found they work great in the dark. The Sokkia iX I was using for a while would always lose lock when up close at a bit of vertical angle (in an excavation) but in the dark it was fine.?ÿ
Good to know, hopefully I don't need to do it too often though!
