Mz1557

[+ddnutzy]

I finally recovered the station and all four of the reference marks for MZ1557. All of the RM's were a mirror image of the aizmuths in the discription. RM1 was 189 instead of 09 degrees, RM2 286 instead 106, RM3 41 instead of 221 and RM4 131 instead of 311 degrees. Mins and secs not included . THe station and rm's 1,2 and three were easy to find but I had to scratch around for RM4. Four had not been recovered since 1901 I think.

[+Black Dog Trackers]

This must be an example of what a SurveyTech said long ago in this forum - some surveyors were using some convention where N and S are reversed so that N is 180 and S is zero. I consider such a convention a case of severe wackiness, but apparently some surveyors used it in the past.

[+ddnutzy]

MZ1481 was another bm with reversed azimuths. That was the first one that I found like that. There are two more but I don't remember the pid's

[evenfall]

Black Dog,

 

I have to laugh but yes. Whacky as it seems, A "Geodetic Azimuth" was considered to be in the direction of due South until the NGS changed the practice officially to North in the 1970's. If you talk to people at the NGS about it, especially those who have worked in the field they will laugh and probably tell you a story or two about a kind of personal hell they have endured because of that very quirk! :-) You have to pay rapt attention or the math will go off somewhere... What I mean to say is that back in the day, an azimuth mark or even an azimuth spoken of was to be considered to be south of the Station in question. This is why, for the most part, you will find most all the first order triangulation stations, and I say first order because they were usually the stations that would have them, with Azimuth Marks set, will have them set to the south of the station if set prior to the 1970's, most of the time.

 

Just a quirky practice that once was but no longer is. Either way it still has it's legacy.

 

It is sort of cool, and you will see for yourself if you pull the datasheet for MZ1557 that the RM's are found in "geodetic azimuth" and there is the answer to the mirror image. All Geodetic Azimuth means is in the direction geodetic surveyors commonly assign the azimuth for the record. In reality you can assign any direction as an azimuth, but common practices kept everyone thinking in a uniformly standardized way. It is a good thing to keep in mind when searching for RM's that seem to be hard to find. A geodetic azimuth when based on the date of monumentation can mean that the bearings will be in reverse of standard thinking and modern practices.

 

What ever happened to Survey Tech anyway? He hasn't posted in about 18 months...

 

Oh by the way Dave, Peaked Mt 2 RM4 Was found last in the 1977 recovery. You are the first since then.

 

Rob

[+Black Dog Trackers]

evenfall -

 

Thank you for the edification on this issue and for the valuable keyterm "geodetic azimuth" to watch for.

 

I really do wonder whatever happened to SurveyTech. I enjoyed working with him and miss his excellent and informative posts.

[Bill93]

Glad to learn a new term, "geodetic azimuth".

 

Actually, a 180 degree rotation is NOT a mirror image.

[DaveD]

The azimuths to the reference marks listed in the body of the description for MZ1557 are the azimuths determined from south which was the convention of the U.S. Coast and Geodetic Survey (now NGS) prior to the adjustment of the North American Datum of 1983 (NAD 83), when the reference was changed to north. Note in the NGS data sheet the "box score" has the directions reversed to north. South was initially used in the geodetic community in the U.S. because it is commonly used in astronomy since there are more stars on the south side of the meridian in the U.S. than on the north. NGS changed to north azimuths for NAD 83 because the common use in the U.S. surveying community was to north.

[+ddnutzy]

Thanks to all of you for your help on this subject. I had no idea what was going on when I ran into these rm's that were reversed. Now that I know, it will make the searchs a little easier if this problem comes up again.

 

Dave

[+Colorado Papa]

I really do wonder whatever happened to SurveyTech.

Last Visit: Sunday, July 06, 2003

[+Black Dog Trackers]

Last Visit: Sunday, July 06, 2003

Yeah, I know. I tried emailing him a couple months ago. No answer.