Inventing a GPS waypoint for a DP?

John Bell wrote:

The ODP out of MMK for runway 36 reads, "climbing left turn to 1800 via
325° bearing from MMK NDB before proceeding on course".

It seems to me that the easiest way to fly that with a GPS would be to
create a user waypoint which is a 325 bearing, 20 (or whatever) mile
offset from MMK NDB and insert that into your flight plan before the
first real waypoint in your clearance. Take off, track to the user
waypoint until reaching 1800, then do "direct" to the next waypoint.
Can anybody think of a good reason not to do it that way?

There are a couple ways of navigating this with the GPS, The best method
depends on what kind of GPS you are using. Here is my $.02 worth of
opinion:

If you have an IFR certified panel mounted GPS, I would execute a direct MMK
and use the OBS mode. If I read the AIM correctly, it appears that you
would have to have the CDI mode set to terminal sensitivity (normally 1 to
1.25 nm) to use the GPS for navigation without having an ADF (AIM 1-1-19f,
http://www.faa.gov/ATpubs/AIM/Chap1/aim0101.html#1-1-19).

All the IFR units are 1.0 mile in terminal. 1.25 is in some VFR-only units. The
procedure requires you set the unit to terminal mode in an RNAV DP.

Those VA legs (leg to altitude) expect you to use the OBS mode or some similar
open track mode.