GPS question

[ae708]

How accurate is the speed function on a GPS? Is it likely to be more accurate than the boat's speedometer?

 

[DJ]

Re: GPS question

You can take the GPS speed as absolutely accurate.<br /><br />I have no use for boat speedometers. I do not even install them in boats that I restore anymore, after the advent of inexpensive GPS.

 

[ae708]

Re: GPS question

Thanks DJ.... that's about what I was thinking.

 

[acc]

Re: GPS question

GPS is accurate BUT it gives you 'land speed'; that is, speed relative to the earth.....If you are trying to troll at a given speed, the speed wheel on most depthfinder transducers is more accurate at giving 'water speed'......When fast tides or currents exist, this difference can be pretty significant....

 

[Drowned Rat]

Re: GPS question

The interesting thing about GPS's is that they only recognise 2 dimensions. Since boats are on water which is perfectly level, GPS speed is very accurate, save for the units update speed during course and speed changes. Where you run into problems with GPS speed and distance measurement is when you operate in 3 dimensions such as hiking in the mountains or flying an airplane. If you hike up a hill with a 45 degree slope at 5 mph, the GPS will only show 2.5 mph. Likewise if you actually walk on that same hill 2 miles, the GPS only thinks you walked 1 mile. GPS units recognise side to side, back and forth, but not up and down. If you take off in a helicopter straight up at 20 mph and travel for 2 miles straight up, the GPS will read 0 MPH and 0 distance travelled. They do have their limitations.

 

[JB]

Re: GPS question

Don't agree, Drowned Rat.<br /><br />GPS operates in three dimensions; four if you count speed as a dimension.<br /><br />Any good GPS displays altitude (height) as well as coordinates. It also uses changes in altitude as well as coordinates to calculate speed in three dimensions.<br /><br />The relationship between the one dimensional speedometer in my truck and the GPS (Garmin 162) does not change when ascending, descending or traversing level ground. If the GPS measured and calculated in only two dimensions the calculated speed would change when changing altitude.<br /><br />A good aviation GPS will display ascending and descending rates as well as straight line, three dimensional speed.

 

[18rabbit]

Re: GPS question

Drowned Rat isn’t totally off base. A lot of GPS require setting the mode; boat, car, foot …whatever. In ‘boat’ or marine mode height is assumed and any calculation for such is not performed. He is correct; typically, GPS will not calculate height on boats.

 

[Peter J Fraser]

Re: GPS question

JB et all<br />I have a buddy who has been using GPS in his aircraft (Cherokee 235) for over 10 years and wont fly without it.<br />The amazing thing about it here was that at the time nearly all instructors were trying to turn pilots away from using the technology.<br />I have been working on developing an GPS asset tracking system for the past couple of years in conjuction with a local manufacturer. My usage is for an earthmoving rental fleet of over 1000 pcs and we have been utilising it for data retrieval and to locate the equipment if it gets stolen. That alone is a rising problem on its own.<br />The end uses for the system are endless.<br />I have applications working in Motorsport and Offshore powerboat racing as well.<br />Good Fun <br /><br />Regards<br /><br />Peter

 

[Drowned Rat]

Re: GPS question

Sorry JB but you're wrong, I understand what you are saying about accent rates and total accent distances being tracked with a GPS, however these calculations are a separate function from the Speed and Distance function of the GPS. They are not integrated. Some GPS use a satelite signal to calculate altitude, but they are woefully inaccurate. Some GPS come with a separate altimeter built into the unit, and although they are dead on accurate and although the GPS displays all the neat altimeter information like accent rates and whatnot, it is a separate tool from the GPS. Every calculation that a GPS makes, it makes from it's 2 dimensional position on the earth, ( lat. and long.) The rate of accent, descent, etc.. are a function of a separate gauge, the altimeter. If you are flying from Boston to Los Angeles and you take off from sea level, accend to 30,000 feet, and then descend back to sea level to land, The GPS knows that you climbed to 30,000 feet and it knows how fast you climbed to 30,000 feet. BUT! it does not take the vertical distance into account when figuring how far it was from Boston to LA! And it does not figure that extra distance when calculating your speed across land. You just travelled 12 vertical miles on top of the 3000 miles across country. The GPS will only read 3000 miles not 3012. If you are using a GPS that "Guesses" it's altitude from triangulation from satelite signals, I wouldn't go flying with you. You'd smack a mountain in no time. <br /><br />BTW, speed is not a 4th dimension.

 

[18rabbit]

Re: GPS question

BTW, speed is not a 4th dimension.

Marilyn McCoo is in the 5th Dimension.

 

[Drowned Rat]

Re: GPS question

I had to look that one up 18rabbit

 

[ThomWV]

Re: GPS question

DrownedRat,<br /><br />Nope.<br /><br />They measure in 3 dimensions, all of them do it and always have right from the start. Virtually all GPS units take into account altitude and will measure speed up from the surface just exactly as well as they will measure speed across the surface. The only time they don't take altitude into account they will tell you that you have a two dimensional fix. As soon as they have 3 birds in sight they will go to three dimensions. <br /><br />18Rabit, what GPS are you talking about that has you input the use (walking, boating, aircraft, whatever) you are making of it? I have never seen that on any recreational GPS.<br /><br />I don't know about you guys but I've owned at least a dozen different GPS units and used a lot more than I've owned. I just can't seem to figure out where a lot of this GPS information is comming from.

 

[Drowned Rat]

Re: GPS question

ThomWV, If the GPS is using the triangulation method to measure altitude (BTW it takes 4 satelites to get a 3D fix not 3) or if the unit has a spearate altimeter (Much more accurate) it will display your vertical speed ONLY. Try this example. You're in a helicopter and you take off and go straight up. The altimeter function will show your accent in terms of feet per second which of course can be coverted to miles per hour or any of a dozen different measurments of speed. However your speed over ground will read 0. Now what if you take off in this helicopter and fly up from the ground and forward at a 45 degree angle. What do you think the GPS will show? It will give you TWO readings, your rate of accent and your speed over ground. Two different functions. It will not tell you how fast you are traveling through the air, they don't work that way. GPS's use your current speed over gound and your current location to determine an ETE to a waypoint. This would not be possible if they worked the way you imply. If you are flying on anything other than a level course, your speed will be read only as fast as you are moving over the ground which will differ from your air speed or what have you. If they took into account your vertical accent and gave you your true "airspeed", waypoint data would be WAY off as to time enroute and distance to the waypoint. Think about it, it makes sense. The distance between point A and point B is Fixed. Just because you take off and fly up form point A and then down to point B, the distance between the two does not change, even though you have travelled a greater distance. Good GPS's don't even rely on satelite triangulation to determine altitude, they incorporate a built in altimeter to display that information. I too have used many GPS's and have used them in a good number of different applications, and I know their limitations. GPA's DO NOT take into account altitude when determining a position. If you are a climber and you have a GPS hooked to your belt and you climb a vertical cliff 1000 feet high. When you reach the summit the GPS will show that your location is the same as before you started your climb, likewise it will show your distance travelled as 0, and if you were to look at it while you were climbing, it would show your speed as 0. The altimeter function would give you a rate of accent, but the GPS does not directly interface with the altimeter to give you speed and distance even if the GPS itself is aquireing an altitude fix. They are two different instruments giving you two different readings.

 

[sloopy]

Re: GPS question

I agree with Thom 90% I have owned 4 gps; a Garmin Etrex, a Magellan Blazer 12, A Garmin gps 12, Another Etrex and my newest one is a Garmin GPS 76s. ALL of my GPS will tell you the speed you are traveling, not the speed over ground. If the GPS is logged on and you hop on a rocket ship and go strait up to thirty thousand feet at 300 miles an hour, the GPS will tell you that your max speed was 300 miles per hour. <br />A GPS needs three satellites to determine you position on land, A FOURTH satellite is needed to determine you altitude. BUT I have driven up hills with only three satellites and the GPS was still able to tell me the same speed the odometer was displaying, it did not however give me an altitude.

 

[JB]

Re: GPS question

Well, perhaps different GPS units have different capabilities and software. They certainly have different features.<br /><br />Any multi satellite fix is three dimensional, whether the unit uses and displays the third dimension or not. <br /><br />Ask any old time aircraft celestial navigator who had altitude-induced errors in his sextant star shots, and took flack for fixing his position "in a cocked hat".<br /><br />The multiple distance "lines" will converge at a point in space, which is at certain Lat/Lon coordinates and at a certain altitude at, below or above sea level.<br /><br />Understanding of the theory of celestial navigation and of GPS positioning will clarify the fact that a position calculation that does not include altitude will have progressively greater error as altitude above sea level increases, just like navigating by the stars did when I did it from aircraft, lo, those many years ago.<br /><br />If your GPS gives answers in only two dimensions it is shorting you an important feature.

 

[Drowned Rat]

Re: GPS question

I give up

 

[sloopy]

Re: GPS question

DrownedRat said:<br /> "Some GPS come with a separate altimeter built into the unit, and although they are dead on accurate and although the GPS displays all the neat altimeter information like accent rates and whatnot, it is a separate tool from the GPS. Every calculation that a GPS makes, it makes from it's 2 dimensional position on the earth, ( lat. and long.) The rate of accent, descent, etc.. are a function of a separate gauge, the altimeter" <br /><br />NO! An aviation GPS will display the altitude that the satellites are telling it, UNLESS it is one of the fancier units acts like a glass cockpit and in that case it takes in altitude information from a separate altimeter. Aircraft have many ways of finder their altitude. Most aircraft use a simple barometric altimeter, and the fancier ones may have the barometric altimeter, a sonar type altimeter, and a GPS.<br />MY GPS76s (WHICH IS NOT AN AVIATION UNIT) has a barometric altimeter built in, it will tell me, my glide slope, feet of descent or climb per minute and altitude. IT can also tell me this with the ALTIMETER switched off.

 

[sloopy]

Re: GPS question

OH by way, one of the ways to calibrate the altimeter is with the GPS

 

[Drowned Rat]

Re: GPS question

All of that is true Sloopy, but rate of accent is different from speed over ground. When you accend or descend, your actual speed is greater than your speed over ground. The steeper your accent or descent, the greater the variance. The GPS displays your speed over ground only.

 

[Drowned Rat]

Re: GPS question

Sloopy, if your calibrating your altimeter with a GPS fix, put me down as your beneficiary.