Does a waterproof phone pouch or dry bag affect GPS accuracy on a hike?

The short answer

A dry, snug waterproof pouch does almost nothing to a GPS signal on its own, because the material — usually a clear vinyl or TPU film — is non-metallic and essentially transparent at the frequency GPS uses, the same as a plain plastic phone case. What changes the picture is water getting between the phone and the outer wall of the pouch: a loose pouch that traps an air gap can let a puddle collect right over the antenna if water works its way in, or if enough condensation builds up inside a sealed bag over a long day. That trapped water, not the pouch material, is what does the damage.

Why water is a different problem from plastic

GPS signals are microwave-frequency radio waves, and the reason a case rarely matters is that ordinary plastics and rubbers barely absorb or reflect radio waves at that frequency — the signal passes through as if the case were not there. Water behaves completely differently: it is a lossy medium at microwave frequencies, meaning it absorbs a meaningful fraction of the signal passing through it rather than letting it by. That is the same underlying reason a GPS receiver cannot get a fix underwater at all. A thin, incidental amount of water does far less than full submersion, but it is not doing nothing either — it depends on how much water there is and where it sits relative to the antenna.

A snug pouch versus a loose one

The distinction that matters is not "waterproof or not" but "snug or loose." A pouch pressed close against the back of the phone, with most of the air worked out before it is sealed, behaves close to how a plain case does — there is no meaningful pocket for water to collect in even if a little gets past the seal over time. A loose pouch, worn on a lanyard or clipped somewhere with slack in the material, gives water somewhere to pool if it gets in at all, and that pocket can end up sitting directly over wherever the antenna is on the back of the phone. The pouch's waterproof rating does not tell you which situation you are in — how it is worn does.

Rain is not the same as a dunk

Ordinary rain falling on a phone, pouch or no pouch, is not the scenario that matters here. A film of rain on the outside of a device is thin, constantly renewing itself, and nowhere near the volume it would take to meaningfully absorb a GPS signal — phones track a position in the rain all the time without any special protection. The concern with a pouch is specifically about water getting trapped in a gap between the phone and an outer waterproof layer, which is a different physical situation from rain simply landing on a surface and running off. A pouch used only to keep a screen dry in a drizzle is unlikely to see this problem at all; one used for a swim or a genuine dunk is a different story.

Kayaking, canoeing and river crossings

On the water, the more common reason for a weak signal is not the pouch itself but where the phone ends up — sealed in a dry bag and stowed inside a hull, a hatch, or the bottom of a pack under other gear, none of which have a clear view of the sky at all. That is closer to a burial problem than a waterproofing problem, and the fix is the same as it would be for a phone buried at the bottom of a hiking pack: a pouch worn on deck, on a life jacket, or clipped somewhere the sky is not blocked keeps the signal path open regardless of how waterproof the pouch itself is rated.

Things that trip people up

• Assuming "waterproof" and "GPS-transparent" are the same property — the material can be both waterproof and radio-transparent, or waterproof with a lining that is not.
• Blaming the pouch for a weak signal when the real cause is where the phone ended up — buried in a hull or under other gear, with no view of the sky at all.
• Not distinguishing rain, which rarely matters, from a genuinely trapped pocket of water, which can.
• Assuming a snug pouch and a loose one behave the same way just because they are made of the same material.

Where TrailCam fits in

TrailCam records with the iPhone's own GPS in the background, the same hardware and antenna every other app on the phone shares, whether the screen is on, off, or the phone is tucked away in a pouch — recording does not depend on being able to see the screen. Rowing and kayaking are both activity types built into the app already, alongside hiking and cycling, for exactly this kind of on-the-water recording. TrailCam is free to download.