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Andrew Corl wrote (in HTML for no obvious reason) on 22 June:

"My question is this, using the sidewalk for a horizon line (and yes it is
pretty level), do I need any other special corrections to my sight besides
dip, index error, refraction, semi-diameter (in other words the usual things
we correct for)?"

With a little experimentation you might be able to use that sidewalk as your
very own personal horizon. What you need is the appropriate correction for
the dip: your elevation above the horizon, plus/minus whatever factor the
sidewalk varies from the horizon. How to work this out? Shoot for the stars!
Find your actual position. Make numerous observations; ideally comparing as
many sights as possible over five minutes with the slope of that body's
actual rise/fall to derive, quite possibly, a better result than any
individual sight. Repeat this process. The average discrepancy indicated
from your(actual position) DR is the sidewalk correction incorporating dip
(the intercept measures error).

Assuming that it does not include some other systematic error. Have I tried
this? Kind of. Sometimes I have found myself making observations from some
peninsula overlooking the sea, and the only accurate data missing was the
height of that cliff. So I worked it out much as explained. It always
impressed me (perhaps I'm easily impressed) that via making observations of
celestial bodies and a process of spherical trigonometry I was able to
measure the height of that cliff accurately enough for my purposes. How did
I know it was pretty accurate? I checked it on a contour map later on.

   

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