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Finding it from the reference marks


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All this time, in the back of mind has been the thought: if reference marks are set, they must be useful for finding the station, but how come I never see the azimuths and distances between them?

 

Finally, I decided to look into this. (Duuuhh!)

 

In the description of the PID is sometimes a section called "Reference Points". In it are distances and aziumths to other stations, etc. Unfortunately, the reference marks are not listed there! Here is an example. However, if you click on the "view original datasheet" link near the top of the PID description, you will see a box made with ascii characters with the stations reference points. In THERE is the distance and aziumth data for the reference marks!

 

There are times when I've found one or more reference marks, but no station, so I couldn't log it as a find. If I'd known about this situation, I might've found the station. From now on, when I see that the station has any reference marks, I'm going to look at the original datasheet link and find the azimuth and distance to each and write them on my printout, just in case I need them. (Yes, I know that magnetic directions drift with time, but it's gotta be better than "18 feet SW of the station".)

 

Note to Jeremy/Elias - next time you import the data, please consider including all the reference points - the ones to the Reference Marks are the ones we can actually use to find the PID.

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I thought you already knew about that, BD. I remember mentioning it at least once. By the way, the azimuths are astronomic so they will not be changing appreciably in our lifetime. Remember also however, that some azimuths are measured clockwise from north and some clockwise from south, so you can get completely turned around if you do not check the relative latitude of the station you are at compared to at least one nearby station to verify which way they are referenced in your area.

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I always try and get all the data....I convert the Meters to feet get the right map datums,for that date,reference angles,azimuth angles,thur not off that far in fact some I did the other day worked real good with the compass I could see all the marks Station ref 1,2 and Azimuth and North Star,As I understand it you are always suppose to recon from the South??That is the Meaning of True Azimuth the Direction of the Plumb line with the Astronomic observation of the North Star? the angle is approximately 0 to 1.5 degrees in in pendulum like movement that is slowly moving to the West at...so many degrees per year.I even reference other map datums as well...Always be prepared,even then you might have to go back again and again and again,and there are still those I have to go back to again....Happy Geotrails

 

WHEN ALL ELSE FAILS *GEOTRYAGAIN* http://www.msnusers.com/MissouriTrails

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sixthings -

 

I've been over there twice. The description's a mess all right! One's been covered up by re-landscaping, and there's some merry mixup of descriptions, coordinates and names of 2 or 3 stations. I believe one is on the railroad you get to by crossing the creek below the dam and climbing the hill (take the second path!). I don't do railroad BMs though, so I gave up at the fence.

 

Nice park though.

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Could you guys elaborate a bit more (in 'simple speak') on the best way to put this extra information to use assuming all you have is a standard compass? As one who likes to find all associated marks, even if they aren't a 'find', I'd love to be able to add this weapon to my arsenal.

 

Greg

N 39 54.705'

W 77 33.137'

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gnbrotz, here's the info -

 

In the original datasheet are the azimuths to reference points. In the example I gave in my first post on this topic, the text of the PID says that Jones 2 is 0.01 miles at 226 degrees. Looking at the original dataset, the more precise data is 21.636 meters and 226.54 degrees.

 

This azimuth, 226.54 degrees is True (geographic) North, as opposed to Magnetic North. A compass will read Magnetic North, of course. The difference between True and Magnetic North is called the declination. Declinations change over the years, and change differently in different places. Usually the difference is only a few degrees.

 

Use this site to find the declination for your area. To use the site, put in your zip code, press the compute button, and read the declination under the underlined D. (The declination number could be either positive or negative.)

 

To use a declination to get magnetic azimuth from true north, use the formula: Magnetic azimuth = True azimuth - declination.

 

An example of use:

In the original datasheet, the reference chart shows an azimuth of 257 degrees from the station to reference mark 1. If we find reference mark 1 first and are having trouble with finding the station, we want to measure the opposite direction, 257-180=77 degrees. The declination at the benchmark's location from the website quoted above is negative 10 degrees. Using the formula above to get magnetic azumuth from true azimuth, 77 - (-10) = 87 degrees. Therefore, we measure 87 degrees with the compass to find the station from reference mark 1.

 

You'll need a compass that measures in degrees to do this, of course.

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Thanks guys! That's the way I was leaning, but the following comments by Survey Tech had me scratching my head:

 

quote:
Originally posted by Survey Tech

Remember also however, that some azimuths are measured clockwise from north and some clockwise from south, so you can get completely turned around if you do not check the relative latitude of the station you are at compared to at least one nearby station to verify which way they are referenced in your area.


 

Why is this done?

 

Greg

N 39 54.705'

W 77 33.137'

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Azimuths can be measured from a reference line either to the north or to the south in a clockwise direction. At different times and places both methods have been used. In the northern hemishpere, land surveyors prefer north azimuths and geodetic surveyors prefer south azimuths. In the southern hemisphere the opposite is the case. Its easy to tell by checking the latitude of the stations involved. If the data says the azimuth from station A to station B is 300 degrees for example, and the latitude of B is greater than A, then B is 60 degrees west of north, or if the latitude of B is less than A then B is 60 degrees east of south.

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