NavList:

Hello Bruce.

Theodolites usually have 30x power scopes, so your limiting resolution should be about two or three arc seconds. However, getting better errors on your position fix than around 1nm is not just a case of using a better instrument to measure altitudes, you also need to increase the accuracy of all the other variables as well

First off, I note that you are only measuring time to the nearest second. One second is worth 25 arc seconds of longitude and so a standard deviation of around 0.25nm of longitude is the best you are ever going to do by measuring time in whole seconds. To improve matters, you need to be measuring time to 0.1 seconds or better. You then need to start paying attention to the difference between UTC (broadcast time) and UT1 (the time standard used for the positions of the celestial objects listed in the various ephererides). This is called DUT1 and value for this are available online.

Your next limiting factor is probably going to be the precision of the ephemerides you are using. The Nautical Almanac (NA) for example, gives the positions of its listed celestial objects to a quoted precision of 0.1 MOA, which is twice as big as what you are capable of measuring.

I guess you are using the Moon as you probably do not have filters to take sightings of the Sun. They do not usually come as standard with theodolites. I prefer to use a filter on the front of the telescope, rather than on the back, as you do not fry the cross hairs in the telescope then. But the problem with a front lens filter is that in can introduce significant angular deviation of the light passing through it, which needs to be measured and known.







At 21:21 22/06/2013, you wrote:

---

Hello:

I decided to start this as a new topic. My goal is to see how precisely I can determine my location using my theodolite (read scale to 3 seconds of arc). My thought process: measure zenith angles to celestial body; do a best fit curve; calculate Hs and Ho; using my exact latitude calculate longitude. The closeness of the calculated value compared to the actual value indicates error. As FER says and I think, error should certainly be 1 nm or less.I've redone my calcs again and still get an error or a couple of miles or more depending on which plotted value I use.

Here is the data: 1. All zenith angles with zero degrees straight up. 2. Uncorrected EDT on June 19,2013; watch is 7 seconds SLOW 3. My location is
42d 20.3 N 71d 48.5 W . Holden, MA my driveway. 4. Sighting on moon UL in early EVENING.

Again ZENITH Angles: 59d 53' 44" {at} 8:26:55; 59d 48' 51" {at}8:28:29; 59d 43' 54" {at}
8:30:16; 59d 40' 30" {at}8:31:28; and 59d 38' 27 " {at}8:32:19 EDT evening.

When I do the intercept process, the LOP goes right through my location (as far as I can tell from plot). Something incorrect in my logic or calculation of longitude. Again, moon UL.

Thank you.

Bruce
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------

: http://fer3.com/arc/m2.aspx?i=124454