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Bill wrote:
"As to distance by height, I have  done many trials with Chicago buildings,
and the Michigan City (Indiana)  light house, cooling tower and smokestack
with an Astra IIIB. I was amazed  at my very poor results, using Bowditch
tables or trig. I won't discount my  technique, but results were often 30%
off our DR/GPS position. "

Frank Replied:
I have a hunch I know what the problem is here. The bases of those buildings
are usually well below the horizon unless you're within just a few miles of
shore. I did some beach sights last week as follows (corrected for IC):
Sears Tower  Altitude: 30.8'
Hancock Tower Altitude: 22.1'
Hancock-Sears Separation: 3d  7' (antennas aligned)
Michigan City Cooling Tower: 10.7'

The  approximate heights of these in feet are: Sears 1450, Hancock 1127,
Cooling  Tower 361. Since my horizon was 4 or 5 miles away, the lower parts
of all of  these buildings were below the horizon, something like 8 minutes
of arc for the two Chicago towers were hidden below the horizon. So instead
of using the altitudes directly, use their difference: 8.7 minutes of arc.

Frank

Good to have you back.

My figures for the Chicago skyline heights seem to match yours. Not clear my
lat/lon are on the money. 361' is the number I have for the Michigan City
cooling tower, but have not been able to get a hard number for their smoke
stack (they believe it is between 450 and 480 feet).

Buildings North to South
Height: Ground to Roof
John Hancock    1,127 ft.
41 53.9       87 37.4
Prudential Plaza   995
41 53.1   87 37.4
AON Center   1,136
41 53.1   87 37.3
Sears Tower   1,454
41 52.7   87.38.1
311 S. Wacker (big light)
41 52.7   87 38.2

Hancock: Rush & Pearson
Prudential: E Lake & Michigan
AON: 220 E Randolph
Sears: W Adams & S Wacker
311 S Wacker: Same
McCormick: 2301 S Lakeshore
Berghoff: 17 W Adams
312-427-3170

I was able to locate a topo map on the web for the Michigan City area, and
it looks like the ground at the cooling tower is about 20 feet above water
level.  For Chicago (since it was a land fill and operates on two levels) I
used a guestimate of 30 feet above lake water level.  In all cases I added
this to the the height of structure from ground to roof top to estimate
height above lake level.

I do believe that were it not for obstructions, I could see the base of the
building from the four-mile-crib area and a bit beyond.  Height of eye 8 ft,
base 30 ft above water level, hence visible for approx. 8 nautical miles.

So far, so good.  But then things fall apart.  The structures above are a
bit inland, and in most cases the bases of the structures cannot be seen
from the water due to obstructions.  The other big problem is the inner and
outer breakwaters which obscure the view of the shoreline.  Therefore I was
using a "horizon" formed by objects a significant distance from the
structures and toward me.  Too many variables I had not accounted for.

I have had some moderate success using angles of separation between the
Sears and Hancock from approx. 6-8 nm away.  Used the sextant for
separation, and a hockey-puck compass for a bearing corrected to true for
Sears.  Of course the angles were high single to low double digits, and
there is just so close a compass will come, so not exactly the best
procedure.  Came within .5 nm of known position, so nothing to write home
about.  Might be fun to try Sears, AON and Hancock with a three-arm
protractor.

I do have much better luck measuring from the roof tops to a clean horizon,
and on an extremely clear day heading back to Michigan City, or over to St
Joe, estimating distance from Chicago by watching building tops of known
height slowly drop below the horizon.  Can't recall the exact number, but
there are approx. 9 buildings poking above the horizon when viewed from
Michigan City.

Which brings me to the question of height on charts of lighthouses etc.  Not
much in the way of tides on the Great Lakes, although lake level can
fluctuate several feet over time. When the a structure's height (lighthouse
etc) is given on a chart, what is the starting point?  Mean low water level
for sea, base, etc?  And is the height to the top of the structure or to the
light?  How does the mariner using height above an observable shoreline or
height above the horizon when the base is below the horizon determine the
actual height above water level of the structure?

Thanks

Bill

   

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