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A Note About BM Coordinates


Elias

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A number of people have mentioned problems with the coordinates being off by a significant amount. There are a couple of possible reasons for this:

 

The first thing to note is that the coordinates on the datasheets are natively in the format DDD MMM SSS.SSS. We convert this to DDD MMM.MMMMM to be consistent with the rest of the site. I bring this up not because of any minuscule error that is introduced in the conversion process, but because of what I'm going to say next. icon_smile.gif

 

Some of the coordinates have been "scaled", meaning that the coordinates have been truncated or rounded. Sometimes this truncation is to the nearest second, sometimes it can be greater. I believe this scaling was an attempt to estimate coordinates for a particular mark when no coordinates existed. I'm sure some of you who have more familiarity with the USGS data and their methods can explain the "why" better than I.... But the point here is that the obviousness of the truncation is lost as we'll always show decimal minutes due to the conversion. This implies a level of precision that doesn't really exist.

 

In another post, someone mentioned that the datasheet also had some text that indicated that the coordinates were rounded and could be off by +/- 6 seconds. This text is called the "Control Text" and can sometimes reveal useful information about the coordinates. Currently, this data is not listed on the mark detail page.

 

I'll work with Jeremy to have both the "SCALED" or equivalent text displayed next to the coordinates as well as including a section which lists the control texts. The combination of both of these pieces of data will allow you to see the accuracy of the coordinates so you can plan your hunt accordingly. icon_wink.gif

 

-Elias

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quote:
I'll work with Jeremy to have both the "SCALED" or equivalent text displayed next to the coordinates as well as including a section which lists the control texts. The combination of both of these pieces of data will allow you to see the accuracy of the coordinates so you can plan your hunt accordingly.

 

Elias --

 

I'm impressed with the way this effort is evolving and the approach Jeremy and you are taking to the issues members are raising. Great work!

 

Gary // Team Grayrun

 

Take care of the land. Someday, you'll be part of it.

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This is one of my favorite warnings to remind people about. Here is a snippet from PID KB0180:

 

quote:
KB0180 ***********************************************************************

KB0180 DESIGNATION - RR MARK 1

KB0180 PID - KB0180

KB0180 STATE/COUNTY- IL/COLES

KB0180 USGS QUAD - MATTOON WEST (1984)

KB0180

KB0180 *CURRENT SURVEY CONTROL

KB0180 ___________________________________________________________________

KB0180* NAD 83(1986)- 39 28 13. (N) 088 26 17. (W) SCALED

KB0180* NAVD 88 - 218.228 (meters) 715.97 (feet) ADJUSTED

KB0180 ___________________________________________________________________

KB0180 GEOID HEIGHT- -32.14 (meters) GEOID99

KB0180 DYNAMIC HT - 218.102 (meters) 715.56 (feet) COMP

KB0180 MODELED GRAV- 980,047.2 (mgal) NAVD 88

KB0180

KB0180 VERT ORDER - SECOND CLASS 0

KB0180

KB0180.The horizontal coordinates were scaled from a topographic map and have

KB0180.an estimated accuracy of +/- 6 seconds.


 

This warning shows up on most vertical control stations. Vertical control markers are not concerned with lat and long coordinates, so they just pull them off USGS 24K maps. Horizontal control stations have obscenely precise latitude and longitude coordinates.

 

rdw

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