# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Distance off with Chicago buildings**

**From:**Bill B

**Date:**2005 Oct 6, 19:40 -0500

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