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Marty,

Take the difference between the height of the mountain 500ft. and your height 163ft + 76ft = 239ft. I get a net height of the mountain at 261 ft. above sea level. Now enter table 9 of Bowditch (1975 edition) Distance by Vertical Angle for 13 nautical miles and you should get 6' which you will add to Hs then correct for refraction and semi diameter Lower limb. This is not a dip short situation. Your visible horizon to the mountain top is above the natural horizon which if used as the reference horizon would give a smaller Hs than if done using a sea horizon. This explains some of the away intercept. Identify the mountain exactly and get a correct distance to it on google earth for entry into Bowditch table 9. A better solution is to just use the 14.4' away as the correction to add to Ho for future observations and not worry about mountain heights and distance off. 

Greg Rudzinski

P.S. An artificial horizon works much better than guessing at mountain heights and distance off.

From: Marty Lyons
Date: 2015 Jun 21, 06:52 -0700

I was doing some sextant practice on land under the following circumstances. Shooting the sun from top of a multi-story parking deck, 76 feet above the adjacent ground. That ground elevation was 163 feet above sea level. The horizon I was using was a low mountain range (to see over local obstacles) about 13 nautical miles away with an elevation of 500 feet above sea level.

I planned on using the dip short formula  Ds= 0.3611(naut. miles)+0.6511(naut miles /h in feet)

Do I need to take into account the elevations of the DR ground and the horizon ground? 

Do I need to account for the difference in those elevations?

Is it even possible to expect reasonable results with this scenario?

Without considering dip short, and just using a height of eye of 76 feet, I got an intercept of 14.4 Nm away.