Alidade

 This is final Alidade for the Exploratorium is based off of the Osborne Fire Finder and my initial cardboard model.  It is 19.5'' in diameter and composed of machined aluminum, machined phenolic as well as glass and acrylic.   It's seen sitting in the Pier 17 window (in SF) on our mock up rail, so the map in this case does not correspond to the viewable landscape as it will when placed in the Exploratorium's new Observatory Gallery at the end of the pier and one story up. Below are a few additional preliminary photos of the Alidade.

 As these were the first photos taken of the finished alidade, the map disk and point ring are made of paper and not the etched and printed aluminum that will be in the finalized exhibit. On the upper part of the map disk you can see a map of the San Francisco waterfront. On the lower part there is the color coded zone diagram which separates the view into various districts. The acrylic ruler that spans the two sights indicates the line of sight as seen in plan (normal map) and in section (zone diagram). The ruler is also graduated to indicate the approximate distance between the observer (at the center of the map) and the landmarks being viewed on the water front map. The small pocket in the lower sight indicates the initials of the landmarks being viewed so they can be looked up in an accompanying book which will provide information on the landmark's historical context and how it relates to the district in which it resides.   Currently, the design of the book is in progress, so is not pictured here.      

Heres another detail of the map and the line of sight rule.

Here is a view of the near sight and the point indicator gauge, the bearing angles are also indicated below the zone diagram.

A view of the far cross hair sight.

Alidade prototype

An Alidade is a device to locate a distant object on a map by line of sight using the 2 sights on the rotating bezel.  This working prototype for the Exploratorium made of laser cut cardboard is based loosely on the construction of the Osborne Fire Finder (a type of alidade) invented around the turn of the last century to aid in locating forest fires.  As the operation of the my model is essentially the same as an Osborne Fire Finder I suggest you watch this video on fire finder operation to learn more about how it's used.  To make it work you need a map with the alidade's location at its center, and to align the map and the compass dial (not seen here) to reality.  


Here you can see the Alidade with the map platen removed, the map platform in the middle where the map platen will be velcroed on and the rotating bezel is on the outside.

With the bezel removed you can see the graduated compass dial.  This was originally etched right into the corrugated cardboard (as seen above) but was replaced with a layer of chip board (think cereal box material) as it etches much more legibly.  

Here you can see the bezel removed from the base of the alidade. In front you can see the bearing window and then the two sights with the range rule between them.  The far sight has as single vertical thread to place over  the object you're sighting like a cross hair.  

Here's the range rule, in this case its calibrated for the particular Bay map seen above, but using a simple ruler and posting the scale as was done with the Osborne Fire Finder would be a good strategy if you're likely to use maps of different scales.  With a permanent range rule like this all maps would have to conform to this scale to be useful. Which direction I'll go with this is not worked out yet given the museum has yet to choose a map or maps for the eventual exhibit.



Here's a detail of the bearing window.  When you set the alidade up you align the map, reality and the compass dial, a thread stretched over window will indicate the exact direction you looking in as long as the alidade is not moved.

I've sighted where the Bay Bridge goes into Yerba Buena Island, from Pier 3 in San Francisco.

Looking down at the map you can see the range rule, which represents the line of sight, passes right over the mouth of the Yerba Buena Island tunnel.