does this compass have adjustment for declination?

It does not have PRESET declination - the mechanism in some compasses allowing you to preset the compass for local declination so that the compass always gives bearings as they relate to true north. Instead, each time you take a magnetic bearing on this compass, you must adjust the compass dial, manually adding or subtracting the local declination to get a true north bearing. Vice versa when you take a true bearing from the map and convert it to a magnetic compass bearing.

Does this have to be near horizontal for the arrow to float freely, or can it be tilted up, say 45 degrees and work?

NO handheld compass can be tilted at 45 degrees to the horizontal and still give accurate readings. Like any compass that rests on a central pivot, you have to hold the compass horizontal to just a few degrees or frictional contact will give erroneous readings - or even stick the dial against the bottom of the compass well. It's not hard to do.

Would this still work if you left it on the dashboard of your vehicle?

As noted by others this is a *magnetic* compass so it aligns it's own magnetic field with the net magnetic field wherever it is located. Any Iron or steel (excluding certain types of stainless steel) will warp the nearby magnetic fields (both the earth's field and the field emitted by the compass needle will be affected). Devices like this are designed to be used in the woods by a boy scout who is not wearing an iron belt buckle. The suitability of any other electromagnetic environment needs to be confirmed if you want reliable readings. Devices placed in cars have to be designed and placed carefully because cars have lots of steel. Below you will find a hilarious photo of a compass next to a phone and the magnetic needle is pointing at the phone instead of north (which has lots of electric fields and possibly magnetic materials, I just tested with my pone and it definitely deflects the needle when the phone is close) The guy posting the photo clearly has no idea that the photo proves his ignorance rather than the malfunction of the compass. Sadly he's given the product a poor rating it doesn't deserve.

Map scale	1:25000
Mounting type	Lanyard Mount
UPC	758399396449 056389096851 824365001980
Global Trade Identification Number	00056389096851
Manufacturer	Coghlans
Color	Black
Size	One Size
Style	Deluxe W/ Lanyard
Length	11.5 inches
Material Type	Plastic
Number of Items	1
Number of Pieces	1
Season	Year-round
Features	compass
Sport	Camping & Hiking
Included Components	Deluxe Map Compass
Batteries Included?	No
Department	Unisex-Adult
Item model number	9685
Product Dimensions	2.54 x 2.54 x 2.54 cm; 50 g
ASIN	B000E22D6I

Coghlan'sMap Compass

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