Mean sea level (MSL) is a measure of the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation. MSL also plays an extremely important role in aviation, where standard sea level pressure is used as the measurement datum of altitude at flight levels.

To an operator of a tide gauge, MSL means the "still water level"—the level of the sea with motions such as wind waves averaged out—averaged over a period of time such that changes in sea level, e.g., due to the tides, also get averaged out. One measures the values of MSL in respect to the land. Hence a change in MSL can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates.

In the UK, the Ordnance Datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921. Prior to 1921 the datum was MSL at the Victoria Dock, Liverpool.

In France, the Marégraphe in Marseilles measures continuously the sea level since 1883 and offers the longest collapsed data about the sea level. It is used for a part of continental Europe and main part of Africa as official sea level.

Chuck Leavell (born Charles Alfred Leavell, April 28, 1952) is an American pianist and keyboardist, who was a member of The Allman Brothers Band throughout the height of their popularity, a founding member of the jazz-rock combo Sea Level, a frequently-employed session musician, and later, the keyboardist for Eric Clapton and The Rolling Stones.

Born in Birmingham, Alabama, Leavell is a mostly self-taught musician. Inspired by seeing Ray Charles in concert (with Billy Preston in the backup band) to pursue a career in the industry, he dropped out of high school. Leavell made contacts with Macon, Georgia-based Capricorn Records, where he met his future wife, and relocated to Georgia.

Leavell joined the Allman Brothers in September 1972, when they decided not to recreate their dual lead guitar sound after the death of Duane Allman, who had died the previous October, but rather to use a different instrument as the second lead. Leavell's work was most prominent on the band's popular 1973 album Brothers and Sisters, and in particular on the heavily-played instrumental "Jessica". However, only one studio album, 1975's Win, Lose or Draw followed, again with Leavell's ebullient piano and keyboard work featured, but with the band in chaos, and on the brink of destruction.

Randall Bramblett (b. Jesup, Georgia) is an American musician and singer-songwriter, whose career as a solo artist, session player, and touring musician, has spanned more than three decades. He has worked with such notable performers as Gregg Allman, Bonnie Raitt, Goose Creek Symphony, Robbie Robertson, Elvin Bishop, Steve Winwood, Bonnie Bramlett, B.J. Thomas, Widespread Panic, and Roger Glover. He plays keyboards, saxophones, guitar, mandolin, and harmonica, and his songwriting is heavily influenced by blues, folk, and gospel music.

Born and raised in southeastern Georgia, Bramblett studied religion and psychology at the University of North Carolina, with the objective of entering the seminary. However, finding inspiration in the music of James Taylor, Carole King, and Bob Dylan, he abandoned his theological studies and pursued songwriting, soon moving to Athens, Georgia.

After establishing himself as a session musician in the early 1970s, recording with Gregg Allman, Elvin Bishop, and others, Bramblett released two solo albums in the mid '70s. He put his solo career on hold, however, touring with bands such as Sea Level in the late '70s, and later with Traffic. In 1998, he resumed his solo recording career after 22 years, and has since released six albums.

David Howell Evans (born 8 August 1961), more widely known by his stage name The Edge (or just Edge), is a musician best known as the guitarist, backing vocalist, and keyboardist of the Irish rock band U2. A member of the group since its inception, he has recorded 12 studio albums with the band and has released one solo record. As a guitarist, The Edge has crafted a minimalistic and textural style of playing. His use of a rhythmic delay effect yields a distinctive ambient, chiming sound that has become a signature of U2's music.

The Edge was born in England to a Welsh family, but was raised in Ireland after moving there as an infant. In 1976, at Mount Temple Comprehensive School, he formed U2 with his fellow students and his older brother Dik. Inspired by the ethos of punk rock and its basic arrangements, the group began to write its own material. They eventually became one of the most popular acts in popular music, with successful albums such as 1987's The Joshua Tree and 1991's Achtung Baby. Over the years, The Edge has experimented with various guitar effects and introduced influences from several genres of music into his own style, including American roots music, industrial music, and alternative rock. With U2, The Edge has also played keyboards, co-produced their 1993 record Zooropa, and occasionally contributed lyrics. The Edge met his second and current wife, Morleigh Steinberg, through her collaborations with the band.