gps accuracy

[h8go4s]

If you wouldn't mind posting pictures of how you mounted your external antenna 6"away from the transom transducer I would like to see it. Never saw that on any boat professionally rigged or otherwise.

My boat is in storage. I can't give you pictures, so you'll have to trust me. Preconceived notions stifle creativity. Think outside the box. Just because you've never seen it or done it doesn't mean it can't be done. My GPS antenna is mounted in the splashwell on a bracket I made. It's well within 6" of the transducer horizontally. Vertically doesn't matter. And "professionally rigged" can mean anything from custom rigged to "hurry up and get this boat done because the ice is out and we have 20 customers waiting". It doesn't mean optimally rigged unless you specify how it's rigged. The installer may put the antenna where it's easiest for him to mount and run cabling. If you want it done right, you often have to do it yourself.

[TruthWalleyes]

OK, we're back to where I entered this thread three days ago:

Originally Posted By: TruthWalleyes

Mine is mounted within 6" of my ducer.

My reply was:

"Mine too. Easy to do with an external GPS antenna."

I feel like I'm talking to a wall. I'm done now.

LOL,

Yeah there is definitely a wall up.

We have external GPS pucks, and the external GPS puck is within 6" of my transducer. Therefore, the cord going from my gps to my display unit could be 100' away, and my GPS points are still within 6" of my transducer! and my display map on my screen is reading where my gps puck is located, not where the display unit is...

If your worried about being that close, y ou shoudln't have gotten an internal gps!

[h8go4s]

I think gps accuracy has a lot to do where you are fishing. I've had errors as high as 40' in some places on some days.

Don't confuse GPS accuracy with map accuracy. My Bird 898 consistently shows position error in the diagnostic screen at 3 to 4 feet, rarely up to 17 feet. The built-in map in my Bird 898 shows my boat traveling on land at Namakan entrance. I haven't found anything like that with my Lakemaster card, but it wouldn't surprise me.

[h8go4s]

That's how some people run into islands in VNP.

Sorry to hear of your accident. I hope no one got hurt.

[delcecchi]

Another source of position error can be the "datum" which is the reference point for the map. I don't know how the datum is handled on map cartidges. But if the gps isn't using the right datum it can cause position errors. My Legend has a whole bunch to choose from. WGS84 seems to be default, but there is also a WGS72 and many more.

Datum

A math model which depicts a part of the surface of the earth. Latitude and longitude lines on a paper map are referenced to a specific map datum. The map datum selected on a GPS receiver needs to match the datum listed on the corresponding paper map in order for position readings to match.

[h8go4s]

Nick, scroll up to the post I made yesterday where I said this:

"I know this is splitting hairs, but the most accurate setup is to have the GPS antenna located as closely to the transducer as possible. With an external antenna, it doesn't matter where the DISPLAY SCREEN is located, does it? What you want is correlation between the sonar reading and the chart reading."

I paid a lot of money for the sonar/gps and Lakemaster SD cards that show 1-foot contours. Why wouldn't I use that equipment's capabilities? Again, I know it's splitting hairs.

[CaptainMusky]

I wasn't saying it couldn't be done I wanted to see how you did it since I've never seen it. I know a thing or two about rigging and was curious how you did it for new ideas. Truth I wasn't worried or scared or anything. I know what I have and what the capabilities are.

[TruthWalleyes]

I don't have a picture, so a quick drawing auto suffice.

FIrst off, i've got a tiller.

Yellow is where i sit.

Brown is my Display

Small red is the GPS Puck

Transducer is blue and sticking out the back.

[SimilarIsland]

Here's my rear Humminbird High Accuracy Antenna mounted above the engine and within six inches or so from the transducer horizontally.

My bow antenna is also mounted within 6 inches or so from my trolling motor transducer. If I mark a fish from the rear transducer,

that waypoint is transferred to my front unit and I can be right over the fish with my trolling motor with only the gps accuracy in error.

If I mounted the transducer only in the middle and my boat were 20 feet long, I would always have the possibility of being 20 feet (2 x 10)

plus the error from the gps.

[CaptainMusky]

Cool setup. I was thinking of getting an external and mount in rear for when I'm at the console. The front works fine with internal since the trolling motor ducer is there.

[SimilarIsland]

Thanks, the internal used at the bow makes a lot of sense by the trolling motor.

[LMITOUT]

I think some are taking the "the spot on a spot" thing a bit too far...

Lake levels, boat movement, GPS and depthfinder processing speed, etc., all add to the inaccuracies and moving a GPS puck isn't going to be enough to make a difference.

Unless you're doing lake mapping, not having your transducer and GPS in-line isn't the end of the world and getting within 10ft of your spot is more than close enough to catch fish.

[SimilarIsland]

If it were only ten feet, that would be a valid point. But what if you mark a rock from the rear transducer one day coming from the west. Then the next day you are coming from the east and looking for that same rock. Let's say you're ten foot from your transducer at the console. The first time you marked it your were ten feet off plus the ten from your gps error which equals twenty feet. The next day coming from the opposite direction you ten more feet because of the console distance and another ten feet (if you're lucky)gps error which makes a total of forty feet. Why would you accept that forty feet worth of error when you could have had 20 feet?

In my opinion, this becomes even more important when you are marking waypoints with your sidescan and going back and trying to find them.

[CaptainMusky]

It really can become a safety issue. There are places I navigate where you have very little room for error and 20 to 40 ft off is dangerous.

[LMITOUT]

It's a moot point. You already have 5,10,15,20+ ft of error in the GPS system when you mark your waypoint, so where the GPS puck is located on the boat means pretty much nothing if you're looking for that type of accuracy. Yes, if it is closer to the transducer it will help, but not to the extent that is being talked about here.

For example, if the puck was on the tip of the bow and you put the bow right over the spot you want to mark, the waypoint already 10' off just because the GPS systems in our boats isn't that accurate.

If you mark that rock in your scenario, did the GPS create the waypoint 10' to the left of your GPS puck/transducer or 10' to the right?

Now, if the GPS system on the boat had sub one foot accuracy then puck placement becomes more important to get that waypoint right on top of the spot you want to mark.

Regardless, the GPS is a tool to get you into an approximate location. From there using the depthfinder takes over to find that "spot on a spot" or avoiding underwater hazards. I just don't believe that not having the transducer in-line the GPS puck is the end of the world and with 10-50ft of error some searching will still be needed to get to that "spot". We're not docking with the International Space Station.

[Capt. H (Ret.)]

I have a Lowrance handheld and a Lowrance with an internal antenna mounted on the dash of my boat. I marked a waypoint on both units and then threw a marker bouy in the lake. I then drove about 2 miles away and followed the GPS back to the waypoint without looking for the marker bouy. When the units said I had "arrived" I looked up and my marker bouy was less than 10 feet away. Close enough for me. Even a walleye will move alittle when they fart!!!! LOL!!!

Mike

[SimilarIsland]

Absolutely!! I fish a system of lakes in Ontario that have 300 foot depths on the left of your boat with a rock sticking out the right. It would take me forever to get through that with a 40 foot error. I can show you many trips I have with three GPS tracks at once where ten feet was the difference between buying a lower unit and not. Let alone finding weed beds, fish, structure, sidescan, etc. I move thirty miles or more through rock strewn areas that I couldn't get to safely without such accuracy.

Can you spend a bunch of time finding rocks and fish slowly moving around with your trolling motor? Yes, absolutely! Do I want to waste my precious time in Canada doin that? Not hardly.

[delcecchi]

You might notice that GPS error changes relatively slowly. That is why WAAS works. If you had done the experiment with the marker buoy only pulled the boat and came back a week later you could have gotten different results.

Personally I wouldn't trust a GPS for navigation if a 20 foot error meant a lower unit.

Here is a whole bunch of data for those really interested.

http://edu-observatory.org/gps/gps_accuracy.html

http://home.comcast.net/~dmilbert/handacc/accur3.htm#link6

[Cicada]

After reading all the responses to dairyman's question, I'm not sure it has been answered, or for that matter, even understood. I talked to him on the ice last weekend and we spent about five or ten minutes on this subject. He is finding that the depth his chip says he is in and the actual depth he is in is nowhere close to the same. One of my experiences in this came last summer on White Earth Lake near Detroit Lakes. The chip/gps location on the map said I was in 19 feet of water. I was actually in 57 feet of water and I had to move towards land about three city blocks to get to 19 feet of water. No other structure in the area that could have hit he 19 foot mark. With a little amatuer visual triangulation, I did appear to be in the location the gps map said I was. White Earth is a fairly large lake for the area, but not a big body of water with enough landmarks to make me confident in my guesswork.

[katoguy]

Cicada, what GPS were you using? What was the estimated position error at the time? What map were you using?

I would be very surprised if you were using a Lakemaster HD map at the time. It sounds like a plain old DNR map or what I used to use (Navionics).

[delcecchi]

Also, some of the lake maps still use DNR data that was collected many years ago, like the 1930's, by CCC or WPA out on the ice with a transit and a chisel. Survey a line, drill holes along it, drop weight on calibrated string. Write depth in notebook. Move to next hole. repeat.

When finished, mail results to st paul. In fact I bet you could print the dnr data for a lake that lakemaster surveyed and compare the results.

[dairyman]

The lake I was on did not have a high def map of it . I was on a smaller lake just south of DL Most of the lakes I fish do not have the high def feature available on the Lakemaster chip that I have. I had checked the position error on the Humminbird that I have and it said + or - 11 feet

[kg2]

That explains it. I also find no value in non-HD maps.

[goblueM]

Also, some of the lake maps still use DNR data that was collected many years ago, like the 1930's, by CCC or WPA out on the ice with a transit and a chisel. Survey a line, drill holes along it, drop weight on calibrated string. Write depth in notebook. Move to next hole. repeat.

When finished, mail results to st paul. In fact I bet you could print the dnr data for a lake that lakemaster surveyed and compare the results.

one of my pet peeves is folks getting frustrated with their lakemap chips and claiming they are worthless, because the actual depth of the lake at their position is not as claimed

Not only do you have GPS error involved, you also have map error. Combined, those things are enough to ensure that the assumed depth (or any given feature) is different than the actual.

And that is assuming that natural conditions haven't changed in the meantime (water level, sedimentation, dredging, or a hundred other things)

GPS and lake maps are a rough guide and shouldn't be relied on for extreme precision or high accuracy

[delcecchi]

Yep. Buy a lakemaster paper map of Lake Vermilion, and a Fishing Hotspots map. Lay them side by side. (unless fishing hotspots has updated their paper maps in the last few years). Huge difference in detail and locations.