+jwahl Posted March 23, 2008 Share Posted March 23, 2008 I started to write something up on this about a year ago, but didn't post it. One thing as a surveyor, you learn is that knowing where you are vertically can be of assistance in finding a BM. I am talking about the vertical control BM's and not so much tri-stations or horizontal control points. For example, if you know the height of the horizontal plane that the benchmark is supposed to be in, this provides a valuable additional dimension that can be useful in minimizing a search area. Well sometimes anyway. Examples I see are BM's along roads with steep banks or hilly roads, BM's on bridge abutments or other structures, etc. The trouble is, or course, that you have to get closer to survey skills and work to establish that plane reasonably accurately. Sometimes just knowing within a foot or two can be useful, but even that level of accuracy is not trivial to obtain. More advanced or dedicated amoung you might have already tried some of this, or acquired something like a rod and level. Those tools could be used in a number of ways to get a close elevation into a BM location. However since BM's are frequently spaced at some interval of distance, it might mean running a level line 1/4 to 1/2 mile or more. In some cases you might consider it though, just another extension of dedication to the search that some might enjoy trying. A homemade rod and hand level might work well enough in some cases. However the idea I was playing with was barometric leveling. This used to be used for reconnaisance surveying for roads, etc. The equipment was pretty expensive still and consisted of a precise aneroid barometer/altimeter and a recording altimeter. The latter would be set as a fixed point and the former taken for a ride, intermediate points where elevations were desired would be read and recorded along with the time. The results of the roving altimeter would then be compared to the stationary one for the same time and a differential elevation or corrected elevation could be determined for the point. That type of equipment is still available, and a lot of it comes up surplus also on Ebay. The company that made such equipment was named American Paulin. What caught my thought process, though was if more modern and less expensive digital altimeters could be used in the same mode. I had acquired a hiking class digital altimeter for about $150 at REI and started testing it as a stand alone device. I have never had a GPS with a built in altimeter, and that would be another extensive topic of discussion as to viability and testing. In this case, it appears to me that it probably only has a usable accuracy of 1-2 meters, even though it can read out to even feet. Changes in location often show jumps of 2-3 feet which indicates to me a possible conversion from integer meters to feet, but it is hard to tell how the thing really works. I have used it to go from a known elevation point next to my driveway to a BM a mile or so away and be within a few feet. What you start to see is that barometric pressure is in a constant state of flux so that even a few minutes of time the altitude set to a BM elevation can change significantly. By the time you travel off to some search area and get back you may have 10-30 feet difference. Now you will have to use some record keeping. There are two approaches I can postulate. One would be time based rate of change, so that you prorate the changes of pressure over the loop run based on time. That would be only as accurate as the assumption that the change in pressure was more or less linear change with time. The alternate would be to have two such devices and do the relative positions somehow. The next step was to look for something akin to the recording altimeter for the fixed station. My search led me to an inexpenive module someone has been building for the model airplane and rocketry folks that can record altitimeter readings at user set intervals and records them in internal memory and can then be dumped to a PC file. The particular module I found was called ZLog. I acquired two of them and the device needed to interface to it. For use in this application it is a bit of a kluge, because you have to rig up a small battery pack for each module. I have not tried this in BM applications, nor evaluated how well it works at this point, so I am just throwing out ideas and wondering if anyone else has been down this road at all. So the way it would work, is that you would start up each module with an approximate or known elevation. Leave one, and take the other on a drive that encorporates maybe one known BM as a check and one point in the vicinity of your search location noting the time since it was started, or maybe several points can be occupied, then return to base. Download the data and do some differential computations. The interesting thing about the ZLog is that it is very sensitve. If you set it on your desktop and initiate it, it will change to a foot lower after being lowered half a foot off the desk. Since it only reads out to the foot, that is about as good as you can get. I can't recall if the digital data is more accurate or not. The next thing is that you notice even more how rapidly pressure changes and results in different altitude reading. Differential comparisons seem like the only way it could be accurate for correct elevation. Another factor that seems clear to me is that how far geographically you try to extend a level run would deteriorate the results the farther you go since weather fronts, etc. are not generally constant over a wide area. If you have a stable high or low, you might be able to go farther distances. Just some thoughts for today. - jerry wahl Quote Link to comment
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