What Is A First-order Baseline?

[+Gonzo-YT]

Hi All,

 

Once again, I seek the collective wisdom of the group. So far, I've learned the difference between triangulation and leveling, first and second-order, height and elevation and all kinds of other cool stuff.

 

One thing I can't track down, what is the difference between a first-order baseline and other first-order triangulation?

 

The USC&GS annual report for 1943 refers to 110 miles of first-order triangulation, and two first-order base lines at Carcross and across the Takhini River.

 

Regards,

Anthony

[Bill93]

Somebody will give you a fuller answer, I'm sure, but the quick one is that in order to locate things by triangulation (measuring angles) you have to start with some points that are a known distance apart so you can find the length of sides in other triangles.

 

So they go out with precision tape or precision electronic distance measuring equipment and carefully measure a baseline (or two) to refer all the rest of the triangles against.

[Z15]

Look at this image of an old triangulations diagram. See the black line at Helps SE Base and Helps NW Base (destroyed)? Its one of the Base Line that are presision measured, usually along a straight stretch of road or R/R (this one along state hwy 69). All the other line are computed based on this measured distance. The meaured distance gives more strenght to the figure and the computations. There is a rule on how far apart B.L.'s are established but I don't have that info.

 

So, you search for marks with BASE in the designation.

 

btw-those highlighted marks I have been to in the past 30 yrs (work).

I wish I could post this complete image but its huge and I can't get the detail.

 

On this larger view, you can see other dark lines indications other B.L.'s...

Edited July 19, 2005 by Z15

[+jwahl]

As to the standard tape. The official way is to send the tape (usually survey type) to NIST and for a particular fee they will give you a calibration report and certification.

 

Some survey supply houses may also be able to do it. For example:

 

Warren-Knight

 

NIST Calibrations

 

- jlw

[Z15]

Whats more, just having a cerified tape is still no guaranty of an accurate measurement. Temperature at time of measure (at both ends of tape, the amount of tension applied to the tape to meaure the distance and weather the tape was supported all contribute to an accuracte measurement. Precision taping is an art, not many will do it correctly and get the most accurate distance.

 

The tape is standardized a 68°F, a difference of 15° will mean a difference of =/-0.01 ft/100ft tape. So you have to record the temp at each measurement and compute the distance. Not an easy task as many would think.

 

Read just how easy it is >>>>HERE<<<<

Edited July 19, 2005 by Z15