By default, LaTeX vertically centres the floats on a float page; the present author is not alone in not liking this arrangement. Unfortunately, the control of the positioning is "buried" in LaTeX-internal commands, so some care is needed to change the layout.
Float pages use three LaTeX lengths (i.e., TeX skips) to define their layout:
\@fptop\@fpsep\@fpbot\@fpsep above each float, so
the \@fptop skip is always followed by a correction for that.)
The LaTeX defaults are:
\@fptop = 0pt + 1fil
\@fpsep = 8pt + 2fil
\@fpbot = 0pt + 1fil
so that the gaps expand to fill the space not occupied by floats, but
if there is more than one float on the page, the gap between them will
expand to twice the space at top and bottom.
Those who understand this stuff will be able to play elaborate games, but the commonest requirement, that the floats start at the top of the page, is a simple thing to do:
\makeatletter
\setlength{\@fptop}{0pt}
\makeatother
Surprisingly, you may find this setting leaves your floats too high on
the page. One can justify a value of 5pt (in place of
0pt) - it's roughly the difference between \topskip
and the height of normal (10pt) text.
Note that this is a "global" setting (best established in a class file, or at worst in the document preamble); making the change for a single float page is likely (at the least) to be rather tricky.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=vertposfp