Lake GPS ???

[WIMUSKY]

Re: Lake GPS ???

Geez fellas...................... :focus: The Op hasn't been active since posting this thread. Maybe he's monitoring it offline. But whatever the case, let's stay on track and not argue amongst each other.......... Thanks....

 

[james__12345]

Re: Lake GPS ???

I don't agree. I'd say it's the exact opposite. Most GPS receivers have no charts included with them. That is why a GPS receiver only costs about $40 or less. Electronic charts are much more expensive. And you need an electronic chart plotter to use them.

Here is a GNSS receiver for $30

GlobalSat ND-100S USB GPS Receiver - Cable / USB GPS Receiver

Here is a fancier GNSS receiver adapted for marine use, with a network interface, and it includes a heading sensor:

Lowrance Point-1 GPS Antenna - LOWRANCE | Marine Electronics

It is $200, but the heading sensor is a really nice addition, since a GPS cannot tell heading.

Go back and read my post instead of just trying to be arguementative. I said the only ones most people have seen, not most of the ones made. MOST people have only ever dealt with the type that mounts to the windshield of a car, MOST of which include maps. Infact, I have never in my life saw a gps unit that didnt include SOME type of map, with the exception of a few very cheap hiking units in the sporting good section of walmart that are sold as a cheap way to mark a starting spot. Even my small hand held magellan unit that I use hunting has a very basic map in it that includes some of the major roads.

But, even to take your approach to it, I would almost guarantee there are atleast 10 to 1 units with maps sold to ones without, because the only gps most people use, are the windshield mount ones in their car.

 

[WIMUSKY]

Re: Lake GPS ???

Go back and read my post instead of just trying to be arguementative. I said the only ones most people have seen, not most of the ones made. MOST people have only ever dealt with the type that mounts to the windshield of a car, MOST of which include maps. Infact, I have never in my life saw a gps unit that didnt include SOME type of map, with the exception of a few very cheap hiking units in the sporting good section of walmart that are sold as a cheap way to mark a starting spot. Even my small hand held magellan unit that I use hunting has a very basic map in it that includes some of the major roads.

But, even to take your approach to it, I would almost guarantee there are atleast 10 to 1 units with maps sold to ones without, because the only gps most people use, are the windshield mount ones in their car.

Why don't you go back and read my post and stop the bickering. Otherwise, this thread will be closed......

 

[NYBo]

Re: Lake GPS ???

To be back on topic (I hope!): Use a combination of your current depth finder and the biggest chartplotter you can afford/fit, and you will be fine as long as you can get a chart for the lakes where you boat. Due to limited space, I went with a combo unit mounted in the instrument panel. Although the base map that came with my Humminbird is okay, I already had the Navionics SD card for my XOG, so I am using that.

 

[bruceb58]

Re: Lake GPS ???

NyBo,

Do your charts for lakes actually indicate depths on them? I know years ago when I was searching for what was available for my Lowrance GPS/Sounder that the only charts I could get with depths was for ocean.

 

[JB]

Re: Lake GPS ???

Gentlemen, Some of you are talking on at least two different levels of technology, and doing it in an unfriendly manner. We Moderators are tasked to quiet down such argument in place of discussion. Please respect the first and most important of our rules:

1. This is a family site. The use of foul or insulting language or the posting of any material not judged suitable by the Moderator Team is forbidden. Disagreements on any topic must be conducted in a polite manner using courteous language.

Thanks for your cooperation.

 

[QC]

Re: Lake GPS ???

bruce,

My Garmin 176C had the Fishing Hot Spots maps and depths were shown on the Powell and Mead charts. Not Havasu or Mohave though. So was hit and miss. I could easily "see" bottom contour though where the data was included. Looked like a Topo Map.

 

[bruceb58]

Re: Lake GPS ???

bruce,

My Garmin 176C had the Fishing Hot Spots maps and depths were shown on the Powell and Mead charts. Not Havasu or Mohave though. So was hit and miss. I could easily "see" bottom contour though where the data was included. Looked like a Topo Map.

Hmmm...I am going to have to check mine out again.

 

[NYBo]

Re: Lake GPS ???

NyBo,

Do your charts for lakes actually indicate depths on them? I know years ago when I was searching for what was available for my Lowrance GPS/Sounder that the only charts I could get with depths was for ocean.

My boat is still under cover, so I can't say for sure. But I'm pretty sure it displays depths on the Hudson River.

 

[The Woo Woo Kid]

Re: Lake GPS ???

:facepalm:You are being obtuse. No one here has said a chartplotter MEASURES depth, just that it INDICATES depth if the chart includes this information.

Well, not exactly. Chartplotters have the ability to measure depths, plot safe courses, show obstructions, shallows.

 

[The Woo Woo Kid]

Re: Lake GPS ???

back on subject.......are the chartplotters smart enoght to take the gps data and adjust the depth indicated for season adjustments?

I've been looking through chartplotters tonight and I saw one that adjusts to current water levels. Hummingbird ?

I believe it used it's altimeter to determine water elevation and then made adjustments.

 

[jhebert]

Re: Lake GPS ???

I've been looking through chartplotters tonight and I saw one that adjusts to current water levels. Hummingbird ?

I believe it used it's altimeter to determine water elevation and then made adjustments.

I don't think that the altitude data deduced from a position fix provided by a GNSS receiver is going to be very useful in finding the elevation of a lake. The dilution of precision of the vertical fix in a GNSS position solution is typically much greater than the horizontal dilution. You might know your horizontal position to an accuracy of 30-feet, but the vertical position solution is probably not as accurate, and might be twice that error. So unless you are satisfied with knowing the lake level with a variation of more than 30-feet, I do not think the altitude obtained from a GNSS receiver is going to be really very helpful.

A barometric altimeter is probably going to be more accurate than a GNSS elevation solution, but it has to be calibrated for the atmospheric pressure variations. I would be very impressed if there were an inexpensive barometric pressure altimeter that could deduce elevation above sea level to a useable precision. A barometric altimeter needs to be calibrated for pressure and temperature. If not calibrated, the barometric altimeter could be off by 100-feet or more. Again, this does not seem like a very useful way to measure lake levels.

Usually on bodies of water where there is a fluctuation in water level, there is some sort of local gauge that gives the level. Reading the current lake level from that gauge would seem like it would be vastly more precise than trying to deduce the lake level from the elevation calculated by a GNSS receiver or a barometric altimeter.

 

[The Woo Woo Kid]

Re: Lake GPS ?

Re: Lake GPS ?

I don't think that the altitude data deduced from a position fix provided by a GNSS receiver is going to be very useful in finding the elevation of a lake. The dilution of precision of the vertical fix in a GNSS position solution is typically much greater than the horizontal dilution. You might know your horizontal position to an accuracy of 30-feet, but the vertical position solution is probably not as accurate, and might be twice that error. So unless you are satisfied with knowing the lake level with a variation of more than 30-feet, I do not think the altitude obtained from a GNSS receiver is going to be really very helpful.

A barometric altimeter is probably going to be more accurate than a GNSS elevation solution, but it has to be calibrated for the atmospheric pressure variations. I would be very impressed if there were an inexpensive barometric pressure altimeter that could deduce elevation above sea level to a useable precision. A barometric altimeter needs to be calibrated for pressure and temperature. If not calibrated, the barometric altimeter could be off by 100-feet or more. Again, this does not seem like a very useful way to measure lake levels.

Usually on bodies of water where there is a fluctuation in water level, there is some sort of local gauge that gives the level. Reading the current lake level from that gauge would seem like it would be vastly more precise than trying to deduce the lake level from the elevation calculated by a GNSS receiver or a barometric altimeter.

Well, sort of. Some of Hummin Birds units are also WAAS enabled. Was reading that makes the GPS readings more accurate, even vertically, and is with 3 ft of accurate.

So, IF that's used to set lake level readouts, it could be within acceptable range for daily lake level use.

 

[QC]

Re: Lake GPS ?

Re: Lake GPS ?

Personally I think lake level should require a manual input. You want that within 1 foot in many situations.

 

[bruceb58]

Re: Lake GPS ?

Re: Lake GPS ?

Personally I think lake level should require a manual input. You want that within 1 foot in many situations.

I totally agree. GPS vertical accuracy is a lot less than horizontal accuracy. With WAAS, accuracy is spec'd at 5'. Horizontal is 3'.

This if of course is assuming you always have a good WAAS signal. Personally, I would not be assuming that is true all the time.

Have read some reports on GPS receivers and the accuracy measured varied from brand to brand and many didn't come close to these vertical accuracies listed above.

 

[jhebert]

Re: Lake GPS ?

Re: Lake GPS ?

Well, sort of. Some of Hummin Birds units are also WAAS enabled. Was reading that makes the GPS readings more accurate, even vertically, and is with 3 ft of accurate.

So, IF that's used to set lake level readouts, it could be within acceptable range for daily lake level use.

The Federal Aviation Administration who operates the Wide Area Augmentation System publishes monthly reports, like this:

http://www.nstb.tc.faa.gov/reports/waaspan43.pdf

If you check the report you see that 99-percent of the time the Vertical Error in WAAS was about 1.9-meters, or six feet. It also notes that these statistics "are compiled only when all WAAS corrections (fast, long term, and ionospheric) for at least 4 satellites are available." That means the best-case situations. In the worst case the error was more like 8-meters or more than 50-feet in the vertical solution.

So the best case is you might know the lake level within about 6-feet. I boat on the Great Lakes, and a variation of 6-feet in Lake Level is something that occurs over about a 25-year period. There are real-time water level gauges throughout the Great Lakes, and you can get the actual water level referenced to chart datum with an update every five minutes from those gauges via a NOAA website. For an example, see

St. Clair River, Lake St. Clair and Detroit River Current Water Levels

The gauge resolution seems to be to about an inch or less. If I were worried about the water level, I would put much more faith in the data from a gauge, one that I could see myself or one that NOAA provides electronically, than I would ever put in the elevation from my GNSS position solution, even if in a precision fix with Wide Area Augmentation System enhancement.

 

[JB]

Re: Lake GPS ???

I have two auto/marine devices called "GPSMAP" by Garmin (276C and 640). Both have built in maps on which position can be indicated. Each also has the capability to install maps of great detail of selected recreational lakes. Those maps display measured depth and other lake features. Neither actually measures depth; both merely display an icon to indicate calculated position.

I discovered that it is an error to believe the depths indicated on these maps. Lake levels vary with time. If the unit indicates that the depth at your position is 5', the actual depth may be 2' or 8', depending on the state of the lake pool. They are pretty accurate indicators of bottom structure, however, which is helpful. You can tell where certain structure features are but you can only tell what the depth was at some time in the past.

If you have a reliable SONAR device, either built in to the same box as the position finder or as separate unit, you can get an indication of actual current depth. By comparing that measured depth with the indicated depth on the chart you can determine any error and use that error to adjust your expectations of actual current depth of the features displayed on the chart.

 

[dingbat]

Re: Lake GPS ???

The charts state "depths noted on chart are for reference only". That should tell you something.

I deal with what can be up to plus or minus 5? change of water levels on 6-hour intervals. To make things worst, the level changes are not constant. You can have a +4? change in water level on one side and a ? 4? of water on the opposite shore. Not to mention a bottom contour that is constantly changing. One good storm and you might as well throw out your map.


How do you deal with it, you stay out of shallow water. For me, anything less than 10 ft. on a chart is ?shallow? and I proceed with caution in those areas. I also set the ?keel offset? on the sounder to accommodate the depth of my lower unit. I do not care how deep the water is from the surface. I want to know how much clearance I have between the outdrive and the bottom.

Technology is a wonderful thing, but you still have to use your head.

 

[jhebert]

Re: Lake GPS ???

A further problem with the proposed but not quite identified chart plotter than can somehow take advantage of knowing the instant lake level is how this data is provided. On every chart plotter that I have seen there is typically no way to enter any sort of offset to alter the presentation of the depths that will be shown on an electronic chart. The data on the electronic chart about water depth is static data. I have never seen any method in a chart plotter that would allow the operator to alter the presentation of depth data based on having entered some sort of offset from the chart datum value. Because of this, I do not see that there is any way to incorporate the instant lake level into the display of depth data from the chart. As I read the initial comment about this, it was suggested that some unspecified model of chart plotter that was perhaps made by Humminbird has this extremely unusual capability.

If there is a particular model of chart plotter, from Humminbird or any other vendor, in which it is possible for the operator to enter an offset to the library of electronic chart stored data about water depth so that the chart plotter presentation shows new and different values for the charted depths, I would like to know where to find this chart plotter and read more about it. As far as I know, there is no such chart plotter, at least not in my experience.

I have seen chart plotters in which the operator can modify the data presented in the display from the stored chart by selecting various options such as USA or International symbology for aids to navigation, and in which certain information contained in pre-defined tiers or layers can be turned on or off for display by the operator, but I have never seen an electronic chart and chart plotter in which all of the soundings in the chart can be changed on-the-fly to represent a new sounding based on some new water level. If such a chart plotter and such digital chart cartography exists, I would be very interested in hearing more about it.

 

[The Woo Woo Kid]

Re: Lake GPS ???

So, any experience as to which digital maps brands are good, better or best ? I will be using them on upstate NY waters, lakes & rivers, with my home Lake being Lake George.

Thanks for helping a Newb out !