# NavList:

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

**Re: Sight reduction using a calculator's POL (arctan2) key**

**From:**Tony Oz

**Date:**2018 Apr 5, 06:07 -0700

Dear Lars, dear Robin!

Yes, it is great to have a formula always giving the azimuth value that does not need any further transformations.

But the good old one, where the azimuth may come out negative, also has its' own merits - if you are going to calculate the way-point coordinates. It is simpler to use the value stored in the register (on my Casio fx-85ES Plus it is the **Y** register).

My cheat sheet goes like this:

- the observer's position
**Lat**and**Lon**are stored in the**A**and**B**registers respectively - the destination's
**Lat**and**Lon**are stored in the**C**and**D**registers respectively - the angle
**t**=**B**-**D**is stored in the**E**register - the formula then:
*Pol*(*cos*(**A**)·*tan*(**C**) -*sin*(**A**)·*cos*(**E**), -*sin*(**E**)); it returns theand*r*in the*θ***X**and**Y**registers respectively - the G.C. distance is then:
*arcsin*(**X**·*cos*(**C**)), the initial azimuth is**Y**(may be negative) - Now for the first way-point at distance Dst° along the Crs° course from the observer's position: I choose some reasonable value for the distance and store it in the register
**F**, the course is already stored in the**Y**register from the first step **Lat**=_{1}*arcsin*(*sin*(**A**)·*cos*(**F**) +*cos*(**A**)·*sin*(**F**)·*cos*(**Y**)) which I store in the register**A****Lon**=_{1}**B**+*arcsin*(*sin*(**F**)·*sin*(**Y**)/*cos*(**A**)) which I store in the register**B**- then I update the angle
**t**:**B**-**D**→**E** - now I just recall the
*Pol*() formula from the history of operations and re-execute it. I write down the current contents of the**A**,**B**and**Y**registers for the way-points' coordinates and next leg course value.

Please comment.

Warm regards,

Tony