GPS as a speedometer

Wondering the other day while slowlllllyyy trolling how acurate the speedometer on a gps actually is?

Theres 2 types of speed, speed over land and speed over water.  Over land they should be fairly accurate.  I've had mine in the car reading near identical to the speedometer.  Speed over water depends alot on current.  When you're cruising with the current you're covering alot of ground but not necessarily paddling fast or you could be paddling your guts out against the current and not going anywhere.  I'm pretty sure most fishfinders in the mid level price range have a speed readout but they only work when submerged in the water, not thru hull.

A GPS will display your speed accuratly over either surface.  It doesn't know that your on land or water.  What your GPS does know is that it's moved and because of that movement, it'll display that movement  to you in units of speed and direction.  As long as the GPS has movement weather your fighting current or riding with it, it'll display your speed, and direction of travel.  What I've noticed with my Garmin II is that it's altitude reading varies while I'm standing still.  I think I understand why, and thats because while I'm not physically moving, the earth is and the satellite is not, so my apparent movenent may be causing the varied altitude readings.
  I may be wrong about the altitude, but, It works for me, and while on the water altitude is not an important function.

I love using my GPS to monitor my for trolling speed and for an accurate measure of how efficient I'm paddling.

It doesn't know that your on land or water.  What your GPS does know is that it's moved and because of that movement, it'll display that movement  to you in units of speed and direction.  

Thats a description of speed  and direction in relation to land.  I didn't really give a very good explanation.  The difference really only applies to trolling.  Say you want your rigs running between 2-3 mph.  If you're paddling against the current and doing 2-3 mph according to the gps(over land) your lures will be running at a speed much greater than the reading because of the water rushing in the opposite direction.  Your bait/lure will be rolling/tangling/running sideways, basically everything its not supposed to do.  Same holds true for paddling with the current.  If you're going 2 mph gps (over land) then your baits may not achieve the right action cause they're moving too slow.  Even though you were traveling  2 mph over land your bait never seen the resistance it needed to run properly.

I picked this bit of info up at a striper seminar on Bethel Island(delta).  Someone asked why the troll bite would only work for them at the top of the tide.  Answer: Cause thats the only time their bb rebels were running true.  They were gauging speed with the gps.  Solution:  Drop your rig in and start the engine (in this case the paddle), get to the speed that makes the lure run correctly and make a note of it on the gps.  That should be the speed you try and maintain.  Periodically adjust it for change in tidal flow or direction.

It's too bad no one makes a current direction indicator.  Wind is the main force that keeps us from easily detecting current direction.  Even a light wind will push you at an angle across the current, unless the two happen to be moving parallel to one another.   In the bay it's simple but in the open ocean who knows.  I suppose one could drop a weight down and paddle in a wide circle until it stays hanging straight down, and then note your heading, but that sounds like too much work.

Makes sense.  I stand corrected.  I do understand the principal of ones rig running apparently faster when going against current. Or, not running properly with current.  My mind is still tossing around the idea that the GPS is going to display a true speed given it's movement relative to the satellites. Land or water. The water current (if any) obviously plays a key roll in how lures will work, given a certin surface speed.  On a lake with little or no water current, I'm going to trust that my current speed displayed on my GPS is in fact my current speed.  Will that displayed speed be the proper speed for whatever lure I might be trolling?  That can only be answered by doing what agarcia suggested.  
  Put the rig in the water start the engin and watch the rig for proper operation!
  Where this is comming from from me is, my experience using my GPS on my 12'er alum boat on lakes only.  (and in my CJ-5 for its speedometer)  I don't take my boat on the ocean, or in rivers with any current.  
  I havent had an opertunity to use my GPS on my kayak yet,  and I can't see that it'll operate any differently that if I was on my boat.
However, instead of just taking for granted that my lure is working without checking, I'll be checking and adjusting as necesssary now.

One thing on GPS- at very slow speeds, the velocity gets less and less accurate.  Since the device only knows your location to some degree of accuracy, if you are moving slowly your location might still fall into the same degree of accuracy and therefore would show you haven't moved. Try it on land- its kinda cool.
On altitude:  Depends on your GPS unit.  If your unit is actually calculating altitude based on meteorlogical data, rather than GPS position- then you can get some errors when local pressure changes rapidly.  I notice it loads here in SF area- where we can have changes locally in pressure.