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Catalina/Capri 25/250 Sailor's Forums

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Humminbird Matrix GPS Problem

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Sloop Smitten
Master Marine Consultant

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USA
1181 Posts

Initially Posted - 02/08/2007 : 11:10:10

Thought I would run this one by the group for comment. Last year I installed a Humminbird Matrix 17 Fish Finder with a Humminbird GPS. The GPS unit is setting on the starboard gunwale, facing up, about 2 feet forward of the stern light. My problem lies with the compass heading I am seeing from the GPS. It seems to be off 4 -5 degrees when compared to my bulkhead compass. I have confirmed my compass is correct comparing it to a handheld compass. The GPS setting is for magnetic north and not true north. This is a switchable setting on the Matrix. For those of you with similar setups, how accurate are your compass readings? Any advice?

Joe Wergers
Utopia
Fleet 7/Oceanside, CA
78 C25 FK/SR #381

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aeckhart
Master Marine Consultant

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USA
1709 Posts

Response Posted - 02/08/2007 : 13:18:02

Joe,

The possibility of your GPS being "off" is remote. My compass reading is more often than not different than my GPS, usually by 3-4 degrees East, depending on my heading. I'm at a loss for terminology here but, magnetic compasses are effected by equipment in close proximity of the compass, like winches. Where is your compass mounted in relation to metal parts on your boat. The handheld may be effected by the same equipment as the bulkhead compass.

There are references out there on how to do this (Chapman Piloting) but, I would suggest removing your compass from the boat and do a basic adjustment away from possible interference - the back yard would work, using your handdeld. Work any difference out using the compensating screws. Then take it to the boat and remount it. Any difference would be caused by something on the boat and needs to be noted on a "deviation table" for each cardinal direction.

Alternatively, you can motor your boat in each of the four cardinal directions, adjusting your magnetic compass to the GPS heading in each direction. Any error you cannot adjust out, should be added to a "deviation table". This should be done on a calm day for accuracy.

When adjusting your compass, make sure you use a nonferrous metal screwdriver so that the compensators in the compass are not effeted. My compass came with a small piece of copper sheeting shaped into a srewdriver for this purpose.

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 02/08/2007 : 14:57:41

http://en.wikipedia.org/wiki/Magnetic_declination

http://www.ngdc.noaa.gov/seg/geomag/magfield.shtml

Trust the GPS

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Stu Jackson C34
Admiral

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844 Posts

Response Posted - 02/08/2007 : 16:25:28

There's course and there's (boat) heading. Suggestions to check with Chapman's is appropriate. Currents could be causing the differences. I find it difficult to read the compass to the same degree of accuracy as the GPS shows.

Edited by - Stu Jackson C34 on 02/08/2007 16:26:57

aeckhart
Master Marine Consultant

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USA
1709 Posts

Response Posted - 02/09/2007 : 06:48:46

The instructons I received with my Suunto compass state that if it has a 3 degree error out of the box, it isn't worth adjusting since it is difficult maintaining a course of less than that in a chop on a small boats such as ours anyway. I agree that it is difficlut to maintain a totally accurate course with a compass however, accurate is accurate. I went through the adjusting/compensating process anyway.
On Lake Superior there are large deposits of iron are all over the place which causes the compass to either swing radically or loose it's accuracy for a time. As a result I tend to use the GPS predominently but track compass error by comparing the two, and note and diferences when I update my cruise notes (every 30 minute to an hour).....just in case I loose the use of the GPS.

Edited by - aeckhart on 02/09/2007 06:49:49

Sloop Smitten
Master Marine Consultant

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USA
1181 Posts

Response Posted - 02/09/2007 : 10:32:33

Thanks for the responses. I have the instructions that came with the compass and will try adjusting it to the GPS. I know entering San Diego Harbor there are landmarks that you can line up to verify a true North or East heading. (Don't remember which one it is.) Maybe they have something similar here in Oceanside. The 3 or 4 degrees isn't much but it's annoying to have two instruments that don't agree and as Al said "accurate is accurate".

Edited by - on

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 02/09/2007 : 13:40:23

Just use your chart of Oceanside harbor. You can easily line up some landmarks and create a "range". Measure the heading from your chart. It is very accurate. Get on the course under motor and autopilot if available. Now you can check GPS, compass, and chart.

This is very hard to do at sea under sail. The boat is yawing so much under wave action. The compass responds instantly. The GPS calculates your heading based on the last 2 positions plus some arithmetic. There is some delay in what the GPS shows. In calm conditions under autopilot the 2 should agree pretty close.

If you think the compass is off 3 or 4 degrees under some headings I would not worry about it (that is so low). I can't steer a C25 to that accuracy.

When in doubt or thick fog, steer to the GPS heading back to your waypoint just inside the harbor.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 02/09/2007 : 14:17:19

If you're going to try to synch the compass to the GPS, make sure you have no current running. Currents are a good reason to rely on the GPS first--it has no idea where your boat is pointed--just where it's actually going.

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The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.