# Parallax to lat/lon/alt conversions

Use the form below to convert (in either direction) between parallax constants, of the ρsin(φ) (rho sin phi) and ρcos(φ) (rho cos phi) form, and latitude and altitude, and/or Cartesian (xyz) offsets from the geocenter. Note that if your observatory already has an MPC code, you can click here for a list of all MPC observatory codes and their positions, or you can enter your MPC code in this form.

Longitude: (use +/-, not E/W)

The longitude is optional. East is positive, West negative. If you provide a longitude, the output will contain links to G__gle™ and Bing® maps of the location, and you'll get a vector offset from the geocenter (x, y, z). Obviously, you don't need a longitude to do the coordinate conversion, but you do need it if you want a map. I recommend it; it's a good sanity check in a situation where errors are easy to make.

Then enter either:

cos sin(phi): Be sure you enter these in the correct order!

rho sin(phi):

...or...

Latitude: (use +/-, not N/S)

Altitude: (should be an ellipsoidal altitude)

...or...

x:

y:

z:

...or...

MPC code:

Hit 'Convert', and you should get values returned in all forms.

Important safety tips : MPC provides parallax constants to five or six places. Beware; that corresponds to 63 or 6.3-meter precision. Depending on what you're doing, you may want an extra digit or two. (It can also help if someone is using the position to determine which scope is which on a crowded mountaintop.)

The altitude should be an ellipsoidal one (a height above the mathematical ellipsoid that is a 'best fit' to the real shape of the earth). Actual sea level in any given place is a more complicated shape, because the earth's gravitational field isn't all that regular. The difference can be as much as 100 meters. You can click here for a converter between ellipsoidal and geodetic ('above sea level') altitudes. Also, you can click here for a pretty good explanation of the difference between an ellipsoidal and an orthometric (above sea level) altitude.

## Contact info

I can be reached at p‮оç.ötŭlpťсéјôřp@otúl‬m. If you're a human instead of a spambot, you can probably figure out how to remove the diacritical marks...