Navigation marker?

[Guest Glenn]

Does anyone know what these markers are used for? Are these aviation markers of some sort?

 

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[Coop]

Does anyone know what these markers are used for? Are these aviation markers of some sort?

Glenn,

 

I believe these are "trig" points for trigonometric mapping. They are points with a very accurately determined location and altitude which can be sighted by a theodolite from a considerable distance (which is why you usually see them on tops of prominent landmarks). It is then possible, using trigonometry, to determine your location by obtaining bearings and elevations from at least two (preferably three) of these points.

 

Probably completely obsolescent now, with GPS giving such accurate positions anyway, although I wonder if these locations are still used to establish a base for Differential GPS to operate from- I understand this system uses a signal generated by a GPS transmitter at a known location to correct the errors in the satellite generated GPS signals. Maybe someone on here with more knowledge of surveying can inform us...

 

If you want to see how accurate such trig measurements can be, try reading "The Great Arc" by John Keay which describes the survey of India in the early 1800's. To give you an idea of what they were able to achieve with theodolites and trigonometry, one of their surveys took them across India from East to West, and when they resolved their calculations from one end to the other, they had a difference of less than 6" in the location of sea-level! The book also describes the discovery that Mt Everest was the tallest peak in that area (and ultimately the tallest in the world) and how it was named. It's a great read.

 

Regards

 

Coop

 

 

[Guest Glenn]

Thanks Coop. Very good explanation

 

 

[willedoo]

Glenn, it certainly is a Trig point marker, they're rarely used these days with the advent of gps surveying.

 

There are cases of gps base receivers located on trig points around the world, but I'm not sure how many are like that in Australia. A general rule of thumb would be if it looks like the one in the photo, it's a trig station only, if it's got an antenna set up nearby, it's a gps base station.

 

The bases are usually set on a predominantly high hill to maximize reception & maintain reliable corrections for ground based gps, so I'm not sure if that's the only reason they would share the same location. I'll quiz the surveyors when I get back to work in a couple of weeks & get the lowdown on it.

 

I'd assume the swiss cheese holes in the plates are to lessen wind resistance. The plate at the top of the marker consists of two plates welded together at right angles.

 

Another question would be whether the two plates are aligned north-south/east-west, I'd tend to think they would be. Here's one in NSW, & also a couple of permanent gps bases.

 

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[Guest Glenn]

Very informative, thank you. Only reason I asked was that someone commented on my photo on facebook asking what it was and I thought something to do with aviation so asked here. Again thank you