on the bottom of the roof?

i am taking down my A pillar to work on some interior lighting,

and i can easily get to my GPS on the dash, and it has plenty of cable.

i had 7 or 8 on the signal strength where it is, in the center forward portion of dash, almost under windshield wipers.

so i could snake the antenna up there, and just use the magnet to stick it on the bottom of the roof

any reason to if i already get good signal where it is?

will being upside down affect the doppler signal?

No, because the metal on the roof will block the signal. It works on the dash because the signal can go through the glass. There's a topic here somewhere in which a guy mounted the antenna on the windshield, behind the rear view mirror, and he got plenty of signal there, because it was just glass. Also, you can mount the antenna up inside the top of the dash board as well, because the plastic in the dash doesn't block the signal.

welp, that's why i asked.

my GPS was about a block and a half off my track last night,

but it was in the same place it done it about a month ago.

the topography must be off in that area.

with 7 of 8 signal strength, it must be good where it is.