Internal GPS

[JTMavs]

I understand that the new HB Helix units have used internal GPS for the last few years. Am I mistaken in thinking that if I mark a brushpile I see on the unit, then go back to look at it with DI or sonar, isn't it going to be off by the distance between the unit and the transducer along with which direction I was moving? Currently my GPS external puck is directly above the transducer. Looking to upgrade to the new Helix but this question lingers.


Sent from my iPhone using Tapatalk

[rnvinc]

You are correct JT ...

Doug explains your exact scenario here ...

GPS and Mapping Information Resource

Internal GPS antennas are best used at the bow where there's only a couple feet between the internal GPS antenna and the xducer on the bow ...

Console units are best served with an external GPS antenna as close to the rear xducer as possible ...

Rickie

[JTMavs]

Makes me wonder why HB went with internal? On the bow is fine, but not for console!


Sent from my iPhone using Tapatalk

[armyman]

You are correct JT ...

Doug explains your exact scenario here ...

GPS and Mapping Information Resource

Internal GPS antennas are best used at the bow where there's only a couple feet between the internal GPS antenna and the xducer on the bow ...

Console units are best served with an external GPS antenna as close to the rear xducer as possible ...

Rickie

Hey, Rickie, regarding your reply to JT's question....with a HB SI unit on the console, if I mark a waypoint, say 50' from the boat and I want to come back to fish it later, how close am I going to get to the waypoint?

Not meaning to hijack your thread, JT but this triggered a question I always had in my mind!

Thanks,
Calvin

[rnvinc]

Hey, Rickie, regarding your reply to JT's question....with a HB SI unit on the console, if I mark a waypoint, say 50' from the boat and I want to come back to fish it later, how close am I going to get to the waypoint?

Not meaning to hijack your thread, JT but this triggered a question I always had in my mind!

Thanks,
Calvin

Error incurred will still happen marking a Waypoint in SI if the GPS antenna is not at the xducer position ... The unit does not know where the xducer is ... The unit only knows where the GPS antenna is on the Earth (from the satellite data received) ...

All Waypoints (regardless of the method of creation and regardless of which screen is display) are assigned coordinates (latitude and longitude) relative to the actual coordinates of the GPS antenna when the Waypoint was created ...

When marking a Waypoint in SI, the unit calculates the distance from the GPS antenna to the "Marked" spot at a direct 90° angle from the GPS antenna ... The unit "Assumes" the SI beams are in that same plane of 90° left and right of the GPS antenna ...

In other words ... If the console unit is using the internal GPS antenna = a Waypoint marked in SI is assigned coordinates as "X" distance from the GPS antenna...and direct 90° left (or right) of the GPS antenna ...

If the transom mount SI xducer is 5ft "behind" the internal GPS antenna ... Then the actual underwater structure marked in SI will also be 5ft "behind" the coordinates assigned by the Unit for that POI (Point of Interest) ...

Rickie

[CrappiePappy]

Rickie .... isn't there also a degree of inaccuracy ?? The general populace units are good to within, what ... 20ft max ??

Also ... if the waypoint mark is expected to be X number of feet from the transducer @ 90deg ... and the transducer signal is actually only telling us how far away from the transducer the object is (at whatever angle, other than 90deg) ... wouldn't that also throw a wrench into the accuracy math ?? Or are the units calibrated to correct ??

[Oldnewbie]

.

Sent from my SM-G935V using Tapatalk

[rnvinc]

Rickie .... isn't there also a degree of inaccuracy ?? The general populace units are good to within, what ... 20ft max ??

Also ... if the waypoint mark is expected to be X number of feet from the transducer @ 90deg ... and the transducer signal is actually only telling us how far away from the transducer the object is (at whatever angle, other than 90deg) ... wouldn't that also throw a wrench into the accuracy math ?? Or are the units calibrated to correct ??

Correct Pap ... Most GPS specs list published accuracy to 8-10ft 95% of the time ... So in armyman's scenario with the internal GPS at the console -- that inherent inaccuracy will be in addition to the error incurred with the difference in where the xducer is and where the internal GPS antenna is ...
----
----

Waypoint algorithms marked is SI are calculated different than Waypoint algorithms marked in any other displayed view ...

It is true that no SI unit can definitely know the depth of the POI being marked out the the side of the boat ... and only knows the the slant range distance to the object ...

This is where the SI unit algorithm calculation uses the Phythagorean theory as discussed by Robert Gecy here ...
Slant Range vs Horizontal Range and Calculating Waypoints

Now adding another iota of inaccuracy - the unit "assumes" it's a flat bottom to the POI... and thus can incur more inaccuracy in the calculation if the bottom slopes up or slopes down from the xducer depth to the POI ...

It is always recommended to physically verify and adjust Waypoints (marked in SI) with the unit's smallest 2d beam (usually the 20° 200kHz 2d beam) ...

Rickie

[wicklundrh]

Zooming in as far as you can go on your map will ensure you get your boat as close the way point mark as possible. Looking at it extended, you can put your boat on the mark and "assume" that you are on the mark. Zoom in and you will see that you are way off.

Also, it really depends on the unit and the type of grid coordinates they are utilizing in reference to their mapping software. A 6 digit grid is only accurate to within 100 meters. An 8 digit grid is good to within 10 meters. A 10 digit grid is good to within 1 meter. In most higher end GPS modules, you are talking somewhere in the middle of 8 to 10. In other words, somewhere in the neighborhood of 3 to 30 feet. Keep in mind that this is in reference to marking a point on TOP of the water. The unit will have to compensate when you mark a spot on the bottom as now you are adjusting for cone angle on your transducer. Some shoot a 9 degree cone, some a 19 degree cone, and some are even higher. 3 to 30 feet is a pretty good guestimate of being on the mark or off the mark.

[armyman]

Thanks, Rickie, wicklundrh and JT. Now I've got to try and digest this!!

Sent from my SAMSUNG-SM-J120A using Tapatalk