Topic: Map Datum???

[Double Digit]

Hey guys got a question about map datum.I was fishing yesterday and the fish weren't coperating so i decided to go look for this wreck that is on my Navionics chart.I got to the area my side imaging was working good and i began the search.I went up and down side to side i covered this area so much that my breadcrumb trail completely covered the area on my chart that you couldn't see the wreck icon no more on the chart.So I'm thinking maybe the map datum that Navionics used and the default datum on my unit might not be the same.How can i find out the datum that Navionics used?

[sonar2000]

Not sure of the navonics datam but the most common is WGS84..I think that is what the HB default is.
Chuck

[RGecy]

Some charts can have different datums.  I am not sure how to check which datum a certain chart is using.   Need to go fiddle with the 1197 for a bit and see what I can find.

In the mean time, I have forwarded thsi on to Greg.

Robert

[sonar2000]

The navonics maps use WGS84.  I dont know if the lake map datum can change but check your HB navigation and see if the datum changed.
  Some folks prefer the NADS27.. You should be able to set your datam preference...
However they must match.  or you will not be at the same physical place even thought the gps position shows the same...
Chuck

[SonarTRX]

Hi Double Digit,

I agree with Chuck: According to the Navionics FAQ, they use WGS84 for all their current charts, except some of their pre-2002 charts:

http://www.navionics.com/MarineFAQs_PlatinumPlus.asp?CategoryID=4&ViewCategoryID=4

If it is important to you, you could do a simple map-validation:

Have you confirmed how well your chart agrees with reality on land? If you can move the boat alongside a structure (pile, dock, boat ramp...) that are also visible on the chart, that would be one way to validate and check what the error might be. (...or if convenient, just remove the sonar and the GPS puck from your boat and do some GPS sampling at some stationary chart-points on land)

Measure the error in northing/easting, and if there are consistent discrepancies(, and you use the correct datum according to Navionics), then apply a Map Offset on the sonar in order to cancel out the errors.


Tore

tore@sonartrx.com

http://www.sonartrx.com

[Double Digit]

Thanks guys now i know I'm using the rite datum next i will do some testing like you suggest sonarTRX that's a good idea.

[Humminbird_Greg]

Even though you may be using the correct Map Datum, the map itself could still be off.  We have seen where some parts of a lake (would apply to coastal waters as well) are off while others are spot on.  This is because the lake “map” is actually composed of possibly many different smaller maps.  One or more of these smaller maps could be off.  Applying a Map Offset will affect every map that the unit uses (internal or from a mapping card), so you have to be a little careful when setting the Map Offset.

I question whether the data that was originally collected has all been converted to the WGS84 Map Datum.  I agree that the newer lakes that have been added or updated, had their data collected using the WGS84 Map Datum; but I don’t know of any coastal water map collection data.  This would all have been taken from government data and would have had to be converted.  Now maybe they did this but after reading their statement I’m not so sure.

That was the map data.  Now what about wrecks and other data?  Were these locations gathered using GPS (and how accurate was it?) or Loran?  I have seen reports where the maps are correct but the locations of special features were not (I could not confirm these as they were not on our local lake).  Also remember that some structures will move with erosion of the sand under them or even due to hurricanes.


DD,
Just make sure that you are using some sort of structure that would not have moved to offset the map.

[mendota]

Navionics map data is notoriously "imaginative".  Borderline fraudulent.  You wouldn't believe how many imaginary structures I have tried to locate, and how many that do exist, that are completely missed on the map.  That's what happens when you have a data point every hundred feet, and then claim you have 1' contours, LOL.

[ITGEEK]

I read that Navionics makes 10,000 changes a year to their maps.
That shows you there are lots of errors in their maps.

[sonar2000]

Remember that all GPS units have the caution....."Do not use this unit as your primary means of navigation". 
This may be more important now that we are seeing "map" issues.
Kind of frustrating.......

I did have another question posed last week from a user who was navigating to a point
and wondered why the unit did not direct him around the piece of land in between him and the point.. .
Chuck