Another interesting document is "The Use and Calibration of Distance Measuring Equipment for Precise Mensuration of Dams" by Kenneth D Robertson of USACE Engineer Topgraphic Laboratories. (ETL-1090, June 1979). I have a hard copy, I don't know if it is available online.
One of the methods he talks about is the method of Ratios. Basically, if you have a known distance in the network, and you measure other lines along with the reference line in a short time period, the atmospheric conditions will be similar, and you can use a ratio of observed to known length to correct the other distances.
I would add that the measurement of temperature, while seemingly simple, is the most difficult part of this for long lines. As I mentioned, pressure varies systematically (and can be computed from nearby weather station observations), and RH has a smaller effect on the distance.
Kent McMillan, post: 398344, member: 3 wrote: I assume that someone has evaluated different algorithms for estimating the line average refractive index from temperature and barometric pressure measured at the endpoints of the line.
you can find approximate formulas for temperature and pressure of the atmosphere for a given altitude. i have, on occasion, when seeking better accuracy, applied the formula to calculate the expected conditions at the other end of a line in order to come up with a midpoint (average) ppm for the line. seemed to work fine when compared to a mean of forwards and backwards measurements along a line.
GeeOddMike, post: 398389, member: 677 wrote: I stay at the Paisano in Maria when visiting.
I'm quite familiar with Hotel Paisano in Marfa, but I'm sure that some of us would like to hear more about Maria.
John Hamilton, post: 398339, member: 640 wrote: Temperature, on the other hand, does change a lot spatially and somewhat with elevation. There is a formula for lapse rate (rate of change with elevation), but it is more subject to error than the almost exact formula for pressure.
A rule of thumb for adiabatic lapse rate is about 2å¡C (3.5å¡F) / 1000 feet.
There is a calibration baseline in my area where about 1/3 of the line is shaded (including the 0 mark) and the rest is in the sun. On a cool but sunny day one must be very clever with the temperature correction on the long lines to get a passing measurement.
As others mentioned, I'd be curious how accurate the temperature reading would be if the instrument is sitting in the sun for very long. My android smartphone has a barometer in it, but I've never checked it against any standard. I'd guess that the sensor itself is probably similar to the one in the total stations.
Kent McMillan, post: 398396, member: 3 wrote: I'm quite familiar with Hotel Paisano in Marfa, but I'm sure that some of us would like to hear more about Maria.
While I would like to say that I stay with my paisana Maria, I am again an automated spell-check victim. The horror...
Shawn Billings, post: 398398, member: 6521 wrote: As others mentioned, I'd be curious how accurate the temperature reading would be if the instrument is sitting in the sun for very long. My android smartphone has a barometer in it, but I've never checked it against any standard. I'd guess that the sensor itself is probably similar to the one in the total stations.
There is an interesting NGS Technical Memorandum exploring how the temperature of the instrument effects the line of sight in the optical NI002 level. This is the Jenoptik and not the much more common Zeiss Ni2.
Of course, you do use an umbrella to shade your instrument...
Mark Mayer, post: 398397, member: 424 wrote: A rule of thumb for adiabatic lapse rate is about 2å¡C (3.5å¡F) / 1000 feet.
There is a calibration baseline in my area where about 1/3 of the line is shaded (including the 0 mark) and the rest is in the sun. On a cool but sunny day one must be very clever with the temperature correction on the long lines to get a passing measurement.
The CBL we use now is great-a little further away than the previous one (Titusville, PA), but this one has pillars rather than ground monuments (no centering error) and is all grass and in the sun (so minimal variations in temperature). It is the Culver Davies Baseline in Ohio.
GeeOddMike, post: 398401, member: 677 wrote:
I don't even use an instrument very much anymore. LOL.
Speaking of levels...we do several bluebook leveling projects a year, either second order class II or second order class I. And a few more that aren't bluebooked but done to the same specs. The main difference between the two as far as I am concerned is that class I requires temperature profiles, which means reading a high and low reading at the tripod for each setup. This uses thermistors (with shield and fans) on the tripod legs. I am not really convinced it makes any difference on the relatively short runs that we do (under 20 km). Class I also requires calibrated rods, class II does not.
When I saw a leveling party in India, they had 6 guys...2 rodmen, an instrument man, a book man (not digital level), an umbrella man, and a guy to carry the instrument (probably that task was beneath the dignity of the I-man).
John Hamilton, post: 398407, member: 640 wrote:
When I saw a leveling party in India, they had 6 guys...2 rodmen, an instrument man, a book man (not digital level), an umbrella man, and a guy to carry the instrument (probably that task was beneath the dignity of the I-man).
That size party is very common with our DOT, it is usually six men if they are using the robot, but only four for GPS work.
Jones, post: 398430, member: 10458 wrote: That size party is very common with our DOT, it is usually six men if they are using the robot, but only four for GPS work.
Six men for a robot? Sounds like our DOT.
I once saw a DOT crew near my office running levels (last year). They did not have a digital level, so they were taking optical readings. They had one guy who used a six foot folding rule to move up and down on the rod at the direction of the I-man, and then he would read the rod where the right angle part of the six foot rule was placed.
Yeah one behind the instrument, one guy had the data collector, three men taking centerline shots. And one guy was just standing there. Not to mention two inmates flagging traffic one of which was not to impressed with the work ethic of the other six. Just being curious I asked the inmate what they were doing and he said they don't do s***. But in the ten minutes I was there they did complete a full leapfrog of the men taking shots.
Thanks to all for your input...its always a humbling experience for me to participate here. There is nothing more important for me then to learn more about surveying, again thanks for taking the time.
I've got to try to remember to prepare my questions better, always seem to leave out key points. For the record I've used thermometers and have kept them shaded. I'm familiar with altimeters but company's I've worked for have not used them. This by no means is an excuse as to why I may not be using them. I can get into trouble trying to keep up with schedules and deadlines so its good to come here and be reminded of the right way to do things.
In any event its always interesting to read your posts, Thanks Again, Paul
