Aerial View Maps

What does a triangle with a dot inside of it mean on old topo maps?

I'm looking at a topo map from 1900 and I see a unfamiliar symbol: small triangle with a dot inside of it. What does it mean and what is the purpose?

Public Comments

1. there should be a legend in the bottom right hand corner or at the top if i recall and it has been years i think it is an abandoned building
2. I guess it has something with horizontal control station .... please check page 14 on the following source link
3. It's a marker showing a specific elevation. You can find it here under CONTROL DATA AND MONUMENTS http://www.compassdude.com/map-symbols.shtml
4. Those are the locations of USGS bench marks, A long metal stake driven into the ground with circular brass/steel top diameter about 3 inches that has the elevation of that specific "point" on the top of it. http://en.wikipedia.org/wiki/Benchmark http://mapserver.maptech.com/mapserver/topographic_symbols/Control_Data.html http://egsc.usgs.gov/isb/pubs/booklets/symbols/