Re: How did Lewis and Clark calculate land elevations.

Apparently Lewis and Clark had no method of measuring their actual
altitude above sea level during their expedition.  

At the time of their journey, the only way of determining the altitude
at a particular location with an instrument was by measuring the
barometric pressure at that location. The aneroid barometer was not
developed until the middle of the 1800s, so they would have needed a
mercury barometer, which would have been very fragile and awkward. 
There is no record in their list of scientific equipment procured for
the journey of a mercury barometer.

The only other method available would have been to use a transit or
quadrant to
survey and compute the change of elevation staring from a point of known
altitude and continuing by line of sight from point to point, measuring
the angle and distance between each, to their location.  This would have
been impossibly difficult and time consuming.  Lewis and Clark did have
a transit for celestial measurements to determine latitude and
longitude, and possibly used it to estimate the height of landmarks, but
this would have the height above the location of the transit, not sea
level, and would also have required knowing the distance to the
landmark.

References:

A description of altitude determination by explorers in the 1840s.
 http://hometown.aol.com/rte395/hypsometry.html

Equipment and supplies taken on the Lewis and Clark expedition.
 http://www.senate.gov/~dorgan/lewis_and_clark/supplies.html