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trmcconn

New DSWorld and AZ marks

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Dsworld has a feature (don’t know if it’s new or old) that lets you pick a state and county and it will launch google earth and fly in above the selected county with every survey mark in that county filled in. I noticed it even includes azimuth marks, which prompts a question: since azimuth mark positions are very roughly known at best, what information does it use to compute their position?

 

Today I managed to squeeze in a little benchmarking after visiting a high school in western New York. The goal was to recover station Boyce (Cattaraugus county NY) PID NC1013.  The evening before I prepared by entering the coordinates for Boyce and its RMs in my handheld GPS. I also tried to predict the most likely location for the AZ mark by drawing a line at the proper azimuth in Google Earth and noting where it crosses a road at approximately the right distance. (In my experience, AZ marks are usually next to roads. If you get to pick where to put one, wouldn’t you put it next to a road?)

 

Boyce was set by CGS in 1935 and recovered by a CGS party in 1964. The recovery report noted “the direction to the azimuth mark was found to be 20 degrees higher than the original direction and after double checking we feel that the original direction was an error.” 

 

The azimuth listed on the datasheet is 91 degrees, but there is no indication whether this is the original figure or the corrected one. Accordingly, I entered two waypoints for the AZ mark: one based on an azimuth of 91 degrees, and one based on 111 degrees. I considered the first possibility the more likely one since the original party had described the AZ mark as Northeast of the station, which would make sense only if the original reported azimuth had been 71 degrees.

 

I did indeed find the AZ along a town road at 91 degrees azimuth from the station. The curious thing is that DSworld plots the Boyce AZ mark at 71 degrees azimuth and a distance of 1.8 miles from the station near a different town road. The 71 degrees makes sense, but the 1.8 miles is way off from both the original survey and follow up estimate of about ½ mile. (I found the AZ at a distance of about .78 mi.)

The question remains: what data is DSworld using to compute the position of this AZ mark? I can only conjecture that it is using some very old database containing the 71 degree figure before it was corrected to 91 degrees based on the 1964 report. But databases didn’t exist in 1964. Did it take 40 years for the error to get corrected? Strange.

 

After the 1964 recovery the only report is one from 2002 by our good friends in the Power Squadron: All marks recovered in good condition.

 

 

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Probably someone read a faint, smudged, or sloppy 9, or low-quality copy, and took the 9 as a 7.   The AZ coordinates were probably scaled like those for elevation marks, and if 71 degrees hit a road at 1.8 miles, those coordinates got entered.

 

Most AZ, reference marks, and temporary bench marks (TBM) have a PID (often starting with B or C) that isn't "published."  If you look them up, though, they do have approximate coordinates and a notation that they have no geodetic quality data.   DSWORLD seems to access those values.   Occasionally you will find one of those marks was measured to the tight standards and has a "real" PID and data sheet, but that is not common.

 

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Yes, a database search on the AZ mark pid yields a fatal error of "No Marks Found" but coordinates are nevertheless given at the bottom of the page with a code of DD meaning "No descriptive text available".  I hadn't realized that AZ marks are assigned scaled coordinates like vertical controls. Could be useful in future searches. 

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