Avionics Bending: Inertial Navigation System (INS) - Part 4B

In my last blog posting I showed you position data from X-Plane displayed on the LTN-72RL CDU. The CDU showed the following:

LAT 47º43.1'N

LON 122º30.8'W

SEA Runway 34R - Incorrect Coordinates on Display!

So, after the blog posting I took the numbers to check the accuracy of X-Plane and plugged them into Google Earth. And to my surprise I was 21.49 miles to the North West of where I was supposed to be!! The simulator was set to the runway end of runway 34R at Seattle Tacoma airport. 

Google Earth Screenshot of Incorrect Data

Wow ... what happened? Could it be that X-Plane is this massively inaccurate!? Well, the answer is very simple, I made a very trivial and very basic mistake. I did not convert the fractions that X-Plane outputs on the longitude and latitude to arc seconds. Hence, what I showed on the display of the LTN-72RL CDU was the fraction and not the arc seconds. X-Plane handed me the data as 

LAT 47.431ºN

LON 122.308ºW

Which of course needed to be convert to arc seconds (multiply the fraction by 0.6) to result in  

LAT 47 25.88'N

LON 122 18.48'W

And, as you can see in the images below that puts me on the runway 34R at SEA with amazing accuracy when I reference X-Plane against Google Earth!

SEA Runway 34 R - Correct Data 

Google Earth - Correct Data

Google Earth - Correct CDU Data Showing Position on Runway 34R