A novice water tower question

[+Team Lawrence]

I logged my first benchmarks today, deciding to start with water towers since I was close to a cluster of three of them and I was pretty sure I could find them even if the coordinates weren't too accurate. Interestingly, I found four water towers within a very small area. Curiously, only three of them appear on the web site. I do suspect that the fourth tower is of a much more recent vintage than the ones listed on the site. Is that why it isn't listed? Did they get tired of making a catalog of towers after a certain year?

[+Black Dog Trackers]

Surveyors use water towers when it's useful for them - they don't necessarily make geodetic points out of all of them. Same with church steeples. If there's 4 in the same place, it's surprising that they would use more than one.

 

In benchmark topics, we like to see which PID(s) you're looking at, preferably using the URL UBBCode feature. It makes our comments more likely to be useful to you.

[DaveD]

NGS, formerly the Coast & Geodetic Survey, positioned landmark objects such as water tanks and towers, radio and television towers and masts, church steeples, airway beacons etc. for many years by the process of intersection from triangulation. Since the vast majority of these objects cannot be occupied with a survey instrument, during the course of making optical observations between other stations in an area, the NGS/C&GS surveyors would sight on these objects to be used as referneces or azimuth marks. If an object could be observed from at least 2 trinagulation stations, then by intersecting the location of the rays of those observations it is possible to compute the coordinates from the known triangualtion stations. Often these objects would be observed from at least 3 triangualtion stations to ensure the integrity of the position. NGS stopped performing conventional triangulation observations in 1984 and moved completely into GPS. Consequenlty objects built since that time would not be positioned in the national reference frame. A description of the process of triangulation can be found in Geodesy for the Layman -- http://www.ngs.noaa.gov/PUBS_LIB/GeoLay.pd

[+Team Lawrence]

Dave: Thank you very much for the patient explanation. I believe that the water tower in question was built in the mid-80s, so that is probably why it wasn't added to the database.

 

For anyone else interested in the file Dave was discussing, this the correct link, Dave accidentally left off the "f' at the end.

 

BDT:

Although I was intending for my question to be more general (Did they stop storing towers in the database at a certain time?), the specific towers I was looking at were GV5462 GV5404 and GV5403. Thanks for the advice.

[+Black Dog Trackers]

Yes, I realized it was a general question and I didn't know the answer to it, but I couldn't help but be curious and want to know where so many water towers were in the same place.