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Name:	Arnold -	Reply
Subject:	Martin pool elevation datum
Date:	11/9/2007 12:38:17 PM

I understand the elevation datum used to assess Lake Martin's pool elevation will change to the use of the the more universally used mean sea level (msl) datum based on World Geodetic System 84 datum, and that this change will go into effect on January 1, 2008. Pool elevation values under this msl datum are 1 foot higher than the currently used Martin datum. Accordingly, summer full pool elevation for Martin will be 491 ft. msl instead of 490 ft. The requested variance elevation for the winter pool will be 484 ft. msl. Hopefully the change won't confuse the friendlies too much in their correspondence with FERC, the COE or whomever regarding their desired Martin pool level regimes after this year.

Name:	Osms -	Reply
Subject:	Martin pool elevation datum
Date:	11/9/2007 8:23:04 PM

Everyone should understand ,also, that the lake is not going up by a foot; they're just making a paper correction to reflect the accurate elevation.

Name:	Arnold -	Reply
Subject:	Martin pool elevation datum
Date:	11/10/2007 2:58:00 PM

Unfortunately, many have been referring to the lake's pool elevation in feet above mean sea level (msl) as Bruce does with the graph posted on this website. This is incorrect. Units for the currently used Martin elevation datum should be referenced in feet elevation. A minor detail but nevertheless a concern as the change in January will likely lead to confusion when folks will see the same units of measurement but a different elevation value. This posting is just to help minimize confusion during a period while a letter writing campaign is underway for prospective rule curve variances.

Name:	Osms -	Reply
Subject:	Roger, Thanks
Date:	11/10/2007 5:16:45 PM

Name:	Ulysses E. McGill -	Reply
Subject:	Martin pool elevation datum
Date:	11/11/2007 1:51:29 PM

I didn't know this.....thanks for the info, as I would have been confused. I always thought the 490 mark was MSL.

Name:	Arnold -	Reply
Subject:	Martin pool elevation datum
Date:	11/12/2007 6:54:03 PM

Most folks were not aware of this. Elevations using mean sea level (msl) as a reference base are commonly used for most points of interest such as lakes and reservoirs. The World Geodetic System 84 (or WGS 84) datum is the most commonly used inland coordinate system that relates points on the earths surface is a fairly accurate coordinate system that approximates the oblate ellipsoid shape of the earth. Actually, average mean sea level is the basis from which elevation is measured.

This may sound redundant but mean sea level is usually the diurnal difference in sea level due to tidal change and the average of that takes into account changes in the diurnal difference due to the change in axis declination of the earth relative to the moon and sun during is annual trek around the sun. The accepted convention is to drop the 'average' and just say mean sea level.

Why is the elevation data for Martin different? When Martin was built, mean sea level was not use as the reference datum for pool elevation. They used the topographic convention of topographic survey datum of that time and it became codified into the Federal Power Commission documentation and more recent Federal Energy Regulatory Commission (FERC) documentation, US Army COE operation manuals and related APCo operations.

If we ever see a full pool here again, it should be stated as 491 ft. average mean sea level. If I see 491 ft. msl, I'll jump for joy anyway. Just get'r up!!!!

Name:	JIM -	Reply
Subject:	Martin pool elevation datum
Date:	11/12/2007 7:32:22 PM

Come on Genus, gonna be busy tonite.

Name:	Arnold -	Reply
Subject:	Martin pool elevation datum
Date:	11/13/2007 9:36:11 AM

"Genus"? Are you referring to a mathematical term or taxonomic nomenclature?

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