# ifinder H2O and Lakemaster Map accuracy?



## bluedevil

I just got a Lakesmaster chip for my Lowrance H20 and was curious of its accuracy for others? I tried it on a local lake yesterday (Five Lakes) and it was not accurate for me at all. I followed the gps to a drop of that I wanted to fish and it said I was right on the edge of it standing over about 10' fow(I was zoomed all the way in). I drilled holes and it was 35'fow. looking at the gps the 30+fow was about 60' further. I then followed it out deeper and it said I was standing over about 30'fow, drilled holes and it was 57'fow. I know they cant be perfectly accurate but thought it would be closer than that..

I do have some concern that my gps may be off. Is that possible? Is there a way to calibrate/verify that it is correct? Any input would help. I have the Manuel but it is buried in a storage unit right now..

Thanks,


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## Shoeman

If you had good satellite coverage, it's the mapware.

I'm sure many lakes do not have good detailed soundings, just general contours. Many of those soundings are decades old and weren't mapped with a GPS to begin with


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## WALLEYE MIKE

I have Navionics in my H20 and on a lake and it was way off too. Showed that i should have been in 20 FOW and actually was still in 5ft. Well off by 100ft or more.


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## Hotwired

If you look at the lakemaster website you will see there are 49 high def inland lakes that have been resurveyed along with the Great Lakes.

http://www.lakemap.com/digitalgpsmaps-lowrance-michigan.aspx

All other lake are most likely scans of the surveys that we done up to 80 years ago.
Those are the same maps available on the MI DNR site.

A lot can change on the bottom in 80 years.


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## bluedevil

Hotwired said:


> All other lake are most likely scans of the surveys that we done up to 80 years ago.
> Those are the same maps available on the MI DR site.
> A lot can change on the bottom in 80 years.


I will have to try it out on one of the recently scaned lakes. If its not in the ball park there I am sending it back. I have maps online that will tell me the same stuff and there free. 

The only reason I was thinking that my gps maybe off was someone posted some gps numbers on the bay were thin ice was reported and it was supposed to be in 19fow, which would be ruffly 3 miles from shore. When I put those numbers in my gps, it said it was 8fow and only 1 mile from shore. I know, don't believe everything you read on the INTERNET, but I have read many of this guys reports and he seems to be a respected bay fisherman..


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## Hotwired

Make sure your GPS is using the same coordinate format that the numbers he gave you were using.

Some use degrees, minutes and seconds.

Others use degree and minutes in decimal format.


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## ENCORE

I purchased the card last spring. Of course its not going to be absolute accurate, but it will get you very close. Those lakes that have just been surveyed will be very accurate. I ran mine around the lake here enough and its very close.
My H20c came today and I've loaded all my ice fishing waypoints. Very easy unit to operate.

I doubt that your GPS is having a problem locking on to your correct position. You can reinitialize the GPS by using the menu. Don't know if you know this, but you may have to hit the menu button twice to get to the GPS menu screen. Go to the GPS set up menu and click on "Initialize GPS" (do outside of course). While there, make sure you have "Use WAAS/EGNOS" checked first. You can coordinate the system in the same menu also. I would suggest using Degrees/Minutes. GPS "Auto Search" is also in the same menu.


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## bluedevil

ENCORE said:


> I doubt that your GPS is having a problem locking on to your correct position. You can reinitialize the GPS by using the menu. Don't know if you know this, but you may have to hit the menu button twice to get to the GPS menu screen. Go to the GPS set up menu and click on "Initialize GPS" (do outside of course). While there, make sure you have "Use WAAS/EGNOS" checked first. You can coordinate the system in the same menu also. I would suggest using Degrees/Minutes. GPS "Auto Search" is also in the same menu.


I did all that but thanks... I doubted that my gps was off but figured I would try it out anyway. 
I was looking over my way points from Saginaw Bay and Higgins Lake and they seem to be right on with the depth (did not have the chip at the time of entering way points). According to Lakemaster those lakes have been surveyed recently, and the local lake here is most likely the 80 year old survey so I guess I will just have to deal with it on the smaller lakes.

Does any one use this chip on Lake Michigan and if so does the depth contours seem to be accurate out there?


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## MildBill

Hotwired probably hit on the issue, but I will elaborate slightly. Satellite data is calculated using decimal degrees. The "whole Earth" is modelled using either WGS84 or ITRF, and that is the basis for the calculations that your GPS produces. Everything else is the result of internal conversion software within your GPS. (I would always capture, use and store data in its most native state - decimal degrees.)

Prior to 1980, (and even for many mapping applications after) the model of the Earth was based upon measurements that were more continental in scope. (I will not bore you with the details of how it worked.) The model that was used for maps then was NAD27. There is a difference in latitude and longitude values reported between NAD27 or WGS84 or ITRF. (You will also see references to NAD83. Consider it to be the same as WGS84.) The variance between values reported in the 1940's and 1950's (the source of the DNR maps) and GPS is affected by that. However, there are several other factors that can come into play if someone was not aware of what they were doing. The original lake maps were surveyed, and there is a factor called "convergence" that can come into play. This really has an impact on maps that were on the edges of the survey system. Also, Michigan changed the state plane projection system dramatically between 1927 and now. Finally, the DNR uses a custom map projection that can be a further source of confusion if someone is not careful. (If that were not enough problems, digital data for "projected" information can vary by around 600' in a north-south direction if NAD27 and WGS84 get confused.)

If all of those were not enough of a problem source, the original data were point locations. Someone had to enter those points and then probably used software of some kind to interpolate those points into contours. How good was the data entry and that interpretation?

What are the chances that someone got confused or trapped by one or more issues somewhere along the line? Probably pretty good.


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## MildBill

I should also add that if you are concerned about the accuracy of your GPS, the National Geodetic Survey maintains a network of locations that are located with various degrees of accuracy. You can go to their web site and locate a known position nearby. They will report its position (WGS84, decimal degrees), and you can use that to compare with the value you obtain with your GPS. They have a pretty big network of known locations, so you should be able to find one fairly close to where you live. (Unless you live way out in the boonies somewhere.)


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## bluedevil

MildBill said:


> Hotwired probably hit on the issue, but I will elaborate slightly. Satellite data is calculated using decimal degrees. The "whole Earth" is modelled using either WGS84 or ITRF, and that is the basis for the calculations that your GPS produces. Everything else is the result of internal conversion software within your GPS. (I would always capture, use and store data in its most native state - decimal degrees.)
> 
> Prior to 1980, (and even for many mapping applications after) the model of the Earth was based upon measurements that were more continental in scope. (I will not bore you with the details of how it worked.) The model that was used for maps then was NAD27. There is a difference in latitude and longitude values reported between NAD27 or WGS84 or ITRF. (You will also see references to NAD83. Consider it to be the same as WGS84.) The variance between values reported in the 1940's and 1950's (the source of the DNR maps) and GPS is affected by that. However, there are several other factors that can come into play if someone was not aware of what they were doing. The original lake maps were surveyed, and there is a factor called "convergence" that can come into play. This really has an impact on maps that were on the edges of the survey system. Also, Michigan changed the state plane projection system dramatically between 1927 and now. Finally, the DNR uses a custom map projection that can be a further source of confusion if someone is not careful. (If that were not enough problems, digital data for "projected" information can vary by around 600' in a north-south direction if NAD27 and WGS84 get confused.)
> 
> If all of those were not enough of a problem source, the original data were point locations. Someone had to enter those points and then probably used software of some kind to interpolate those points into contours. How good was the data entry and that interpretation?
> 
> *What are the chances that someone got confused or trapped by one or more issues somewhere along the line? Probably pretty good.*


I would say the chances are good also cause it sure confused the hell out of me....:lol:

Thanks for the info on the National Geodetic Survey, I will check it out..


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## waterfoul

I have noticed some lakes are definitely better than others. But there is a disclaimer that comes up on your unit: "do not use as your only means of navigation" or something like that. They mean it!!

I've noticed that my Lakemaster chip is a bit more accurate than my Navionics chip for some lakes... and just the opposite for others. It really all depends on the data available to the cartographers that made the chips.


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## tommy-n

I called lakemaster last week and talked with a guy in customer service. First off he was a pleasure to talk to and was very honest. My question to him was how will your michigan inland lake chip compare to my garmin gpsmap 182 with the bluechart map of lake huron

He said, on some lakes it will be very close, the ones we recently surveyed

on other lakes where we used information from the michigan dnr lake maps it may only be accurate to 50 ft and in some cases even 100ft

apparently some of the information they use to make the maps could be up to 80 years old and thats what they had to work with.

obviously the maps that are in blue or red are very accurate, and the enhanced maps can be pretty accurate or 50-100 ft in some cases

I would just like to thank the guy I talked too for his honesty


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## skeeter341

I have the lakemaster pro N chip in an Expedition C and I am really impressed on how accurate it is ,I use it in mainly on the hi def. lakes and I wouldnt be with out it !!! JMO Skeeter


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## tgafish

bluedevil said:


> Does any one use this chip on Lake Michigan and if so does the depth contours seem to be accurate out there?


Usually within 5 feet. When running out my sonar doesn't register very well so I use the contour depthsto know when to drop down. All land objects (Piers, channels, etc)have been very accurate.


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