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Nice Photos With Datasheet


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Thanks for bringing this in for us to look at. A nice series of Photos.


I want to call attention to what there is to see in the photos, as this site is considered good for GPS observations. Some hills and trees are in proximity, but the leaves are off the trees due to the season. It is also notable that this Photo Naming Convention is not the one Deb Brown of NGS has asked us to use when submitting our photos to NGS, So please, if you send Photos, Please follow the Naming Conventions they are asking us to use. The criteria is posted in the pinned area of the benchmark hunting forum.


Since the photos are date stamped as March, it goes that the leaves would be off, improving GPS Observability, but what if it were June or July? Are the trees going to factor for GPS Observations? There are trees that will directly shade the area.


This site is a GPS Site, that's a fact, but it is a potentially difficult site for GPS observations during the summer if those trees shade the instrument. In Fact, they could render it not usable at all.


When we try to decide if a site will work for GPS Observations we can think of this as a site that worked in March. But we should also remember the discussions we have had here concerning the angles and proximity to items that can clutter the horizon. If we recover this in July, we may not agree with GPS observability.


If you find yourself in this situation where the judgment call could be iffy, I usually practice the how bad is it test. If it is not too covered in Summer I may say it is observable, but note in the narrative that I feel there could be difficulties. If in the summer, I may note it is not observable, and yet in the winter I feel it could be, I will likely call it a no, and annotate that I feel observation may be possible in the winter.


Is there a hard fast rule we can use? No. Just experience. But we can all get a feel for this, just use your experience and best Judgment. Remember all A and B orders of control are GPS control so we know that the site has been already been observed with GPS. It is just worth factoring the changes of season and even buildings that may have been put in place since original observations. In the same respect, there is no harm done in saying a station would be difficult to use for optical observations to the north if there is now a large building, or even a 10+ foot hedge at close proximity blocking the view. Just call it as you see it.



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Most of us here don't have the experience with professional or geodetic-grade GPS equipment that you have.


Unless the situation is completely obvious -- unsuitable because the station is mounted vertically on the side of a building, or useable because the station is a disk on the roof of the tallest building in town -- I err in selecting the "don't know" button when making my report.



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I've been surprised how bad reception is in the woods in the winter when all the leaves are off. Reflections still seem go cause readings that move 30 feet when you moved 10 and vice versa, despite stronger signals. Not the kind of behavior seen under clear sky. In summer it seemed more reasonable to have problems with trees.


Have you noticed this effect on professional equipment, without the leaves? Is it something that averages out in a long reading period?

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It can stabilize out, and with an RTK GPS Setup, I may set the main unit up in the clear and then go in to the covered area with the rover. It is a DGPS set up in a local manner. I have surveyed deep fill rockeries to be built with heavy woods adjacent to them totally barring the rover from seeing the constellation with that set up, just as an example.


Further the Pro gear has Larger Antennas with much higher gain, and computer processing that dwarfs what a GPSr has... In addition, the antennas are often mounted on a range pole like set up, where I can telescopically raise the antenna up a bit to perhaps improve it's view. Then all I have to do is tell the computer the difference between the Antenna Height and the ground, or if you will, the length of the pole. They are the same thing but that is also the effect it has. In any case the computer now knows where the ground is supposed to be if I was accurate in telling it. In other applications There are D-9 Cat Dozers outfitted with GPS systems that cut grade at full throttle in 2nd gear with GPS telling the operator where the blade needs to be. To the operator it is like watching TV as they do the Job. We still have to keep that Cat knowing where the grade is at when the woods are trying to clutter the view. In other word there are problem areas and we have to find ways to mitigate them.


This is an example from Trimble of a unit that a modern Surveyor uses all the time in many shops... Leica makes Fine equipment too, and I use what I am given. This is just one unit, not the whole set up in any number of configurations, and configurations vary.




If I were trying to do a geodetic survey it could take 2-3 long days of basically setting up over the station and watching the unit while it does it's data collecting. The Unit has a couple days to look at the satellites in the full constellation over a period of time, and since there is always movement in the constellation we get a lot of triangulated solutions. In any set up where GPS is a doable at a location to survey, we want to have a look at birds that are low on the horizon. The incidental angles are large, not obtuse and we get the best solutions from this. This set of photos is showing this setup during the waiting game.


Artman, If you do not know if a location is GPS observable and want to use "don't know" as a fall back, that is fine. I am not insisting a thing, but I am attempting to expose you and others to a situation where it looked iffy but was doable, and I feel that the season had a lot to do with it. We all have strengths in the way we learn. There is a percentage of people out there who learn from examples and that was an example photo of a location that had some problems yet got the job done and the photos illustrated some of the background environment. I simply shared my thoughts on this example as there has been discussion in other threads concerning what constitutes an adequate site for GPS OBS. If anyone gains a little better understanding, then it served the purpose.



Edited by evenfall
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Thanks for the info. I learn so much in these forums.


I am guessing that the NGS criterion is on the stringent side, and that there are cases where the station is technically "not suitable" for satellite use but where you could still get a useable GPS signal.


As a guy who hasn't been out in the field with survey-quality GPS equipment, though, I'll err on the side of "don't know" where there is any question in my mind.



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Thanks again for the great info, evenfall, as always.

This thread brings to mind the time I stopped to talk to a survey guy in the Disney Resort area (CA, the REAL one!). He was actually just outside Disneyland, on resort property. He was wearing one of the Trimble backpack setups, with a good size antenna on a mast over his head. All man portable (no tripods, etc.). He would back up next to a tree (antenna next to trunk), wait about 2 - 3 minutes, save the data, go to the next one, etc. It was summer, lots of leaves on the trees. We talked for a while, as he worked his way down the block. Turns out he was getting "accurate" data (he said with an inch or two) on each tree larger than about 6" circumference, for the Disney landscape department. So now they know where all their trees are, give or take a few inches! Interesting application of GPS!

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I may set the main unit up in the clear and then go in to the covered area with the rover.


Areyou are just mapping?


Do you use OPUS.


Our construction techs would use 1 base and 2 rovers to map and stake out. We, on other the other hand did all the alignment control and property from 2 base sta's. We would establish control stations Use NGS BM's when available) at about 1 mile intervals and process using OPUS. Construction techs would then use our state plane converted to project datum to do thier work.


All survey ties for property and alignment had to be from 2 different base stations to get a an average fix to <2cm.



Retired Survey Tech (State DOT)

Edited by Z15
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Hi Mike,


Quite often I would be running DGPS between my base set up and Coast Guard signals. I try to set my base up over control I have in GIS and is loaded into the gear. Often it is based on what the office has used to work up the job in the first place. They go with local control that they get from County, City, PLSS, Geodetic, whatever. Often the Local Counties are Running OPUS, Some are running CORS and fix their own control to that. In Platt development, we go with what the local municipality wants for Datum, especially when the work will be deeded to them upon completion. Sometimes it is found that NAD 27 fits the PLSS locally better than NAD 83 will and the developer make a political decision. They do not to ruffle too many people over the moving property line thing. Most of the PLSS is done in the office on these and they become points in the GIS, Those points are loaded and I go stake them. For Instance, let's say Point 2763 is a Property corner for Lot 17, and so I stake it, and write the point number as well as the proper nomenclature for the stake. It may include a Lot grade on the point so I drive a Hub and calc the cut or fill to lot grade, and write that on the other side of the Lath. It all corresponds to the building site plan.


Most of the time I am on the rover is Construction Staking. Sometimes, when alone, I can get the accuracy I need without a local base if the DGPS Sigs I get from Coast Guard are good. But you know how that is. the closer you are the better, and hilly conditions can decrease the DGPS Sig quality quickly. Often you just set up the base anyway as you know it will be needed no matter what. I often have GPS running in the Heavy Equipment at some sites, which means a base with what is essentially many rovers, and with difficult terrain it sometimes takes a Permanent base, (temporary permanent, and then I will have to set up another base (tripod) to augment where the main Bases cannot see. The more differential signal you have the better it is, and that is iffy if you don't set up a local base. I have run into situations in hilly situations and late in the afternoon that even with a Base set up, I cannot get enough signal from the Constellation to do my work and I have to go optical. The DGPS is fine but the constellation won't provide an adequate solution. I may have to Augment GPS with construction staking in difficult places as well. I can rove with optical too if I have good local monuments, so I can still go it alone. If not then it is the old fashioned 2 man show.


Other things I may do on large jobs is hop on a quad runner and go take a bunch of readings around the site to topo the thing. We are always needed to help keep track of how the material balances are going on the Cut/fill ratios, as well as materials which may need to either leave and come on to the site. We collect the data and take it back, they load it is the GIS and compare. It gives them an idea as to where the dirt is and if costs are in control.


We just do what we have to to get it done. The ultimate solution is the one that works.



Edited by evenfall
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I hated to work on construction projects. I was in Project Development. I was one tech with 3 PS to work for. Always had to be on the head of a dime or they would not be happy. One even thought it should be closer than that, when we first got our GPS equip. they argued about the 2cm accuracy and wanted it closer.


Our dept set up a state wide CORS system (my range was SUP1 to SUP3) so it was realy easy to set permanent control for our construction techs and the consultants to use. If we did not do this, we would have had every consultant using a different datum and it would be impossible for us to do QA as required.


Was fun while it last but they offered an early out in 2002. My boss (51) and I (55) took it. Let the youngers guys fight with all the Engineer/managers who were constantly telling how to do our jobs. One construction tech who leaves near me has to work alone 90% of time due to managements refusal to hire more people (constrained by gov Granholm). I often see him with his RTKJ base up and running, him with the Rover + in the same day he will be using the Geodimeter Robot TS. During summer they will give him a COOP (college eng intern) to help but most of the time he is just baby sitting as they are a PITA. Have to have a bottle of Mountain Dew and MP3 player or they can't walk and chew gum at the same time. He has 20 more yrs to look forward to. I am glad I don't have to deal with that anymore. Everyone thinks working for the gov't is the best job but those that work there think its the worst big brother was always watching).

Edited by Z15
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