Distance Over Land vs Distance Over Water

[Alaskan Sea-Duction]

So I am setting up my new MFD. And it has Log & Trip and Ground & Trip. Raymarine Tech Support told me one was distance over water and the other was distance over land.

To explain what I am doing, I am setting the trip logs up so I have one that will annotate 3 items.

1. Total distance on the boat
2. Total distance for the summer's cruise (i.e. Our total trip to Alaska)
3. Total distance for the day's cruise.

What is the difference between "Distance over land" or "Distance over Water?"

[Alaskan Sea-Duction]

here is what Chuck from Raymarine Forum told me:

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RE: [CA] Trip Log settings
Welcome back to the Raymarine Forum Tom,

Please click here to view a FAQ addressing this subject. The Log & Trip data item is simply a presentation which will display the Log data item and Trip data item within a single Databox or Data applications panel's data cell. Similarly, the Ground Log & Trip data item is simply a presentation which will display the Ground Log data item and Ground Trip data item within a single Databox or Data applications panel's data cell. Ground Trips 1-4 are analogous to automobile odometers supporting Trip A and Trip B in addition to odometer. Raymarine has gone one step further and creating a total of four Ground Trips.

Q. I am attempting to create a Total distance for our summer cruise. This will not be reset until the end of the cruise, and a daily distance that would be reset at the end of the day's travel.
A. Assuming that you will be are seeking distance over ground, then Ground Log and Trip 1 would seem to be the data items of interest. However, if seeking distance through water, then Log and Trip would seem to be the data items of interest.

[Alaskan Sea-Duction]

I guess it would help if I read the FAQs

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Displaying speed though water and distance through water data
Displaying speed though water and distance through water data

Two types of speed/distance can be displayed by Raymarine's MFDs:
- GPS speed over ground (SOG) / distance (Ground Log, Ground Trip 1-4) over ground and
- speed through water (Speed) / distance through water (Log, Trip).

If you are be satisfied with GPS speed/distance over ground, then no additional sensors need be installed to a MFD which is receiving a GPS position FIX. The MFD would simply need to be configured to display SOG, Ground Log, and Ground Trip within the Databar, within a Data application panel, or within Databoxes. However, if you desire to view speed/distance through water, then a speed instrument or a compatible fishfinder transducer with speed through water transducer (may be included in the depth transducer or the depth transducer may be accessorized with a compatible speed/temp transducer) must be installed.

Speed through water (Speed) is particularly useful in conjunction with SOG in determining through water efficiency calculations as well for trolling and sailing applications. Should the the currently installed fishfinder transducer not support speed through water (i.e. it does not feature a paddle wheel), then speed through water may be added by added to the system by:
- replacing the currently installed fishfinder transducer with a compatible depth/speed/temperature fishfinder transducer
- interfacing the currently installed transducer to a compatible fishfinder speed/temp transducer
- interfacing the MFD to a speed sensing instrument system (ex. i40 Speed/Bidata, i50 Speed/Tridata, i70 MFID w/iTC-5 and instrument speed/temperature transducer).

Most sailboats or vessels operating for extended periods of time on battery would be advised to install a speed instrument as the fishfinder sonars draw more current. Installing a fishfinder sonar module with a fishfinder depth/speed/temp transducer will enable a greater number of features than would be offered by installing a speed instrument alone (most notably bottom/structure imaging and fish location).

If speed is provided by the fishfinder transducer, then the speed must be calibrated (HOME->FISHFINDER->MENU->SET-UP->TRANSDUCER SET-UP->SPEED OFFSET). If speed is provided by an instrument, then it must be calibrated via the instrument display. Failure to calibrate speed will cause the through water speed / log / trip and GPS speed / GPS log / GPS trip values to differ when not operating under influence of current (ex. slack tide, on lake, etc.). Additionally, products supporting single point speed calibration (ex. fishfinding products, i40 Speed/Bidata, i50 Speed/Tridata, ST40 Speed/Bidata, ST60 Speed/Tridata, etc.) will most accurately report speed and trip when the vessel is operating at the speed at which the the speed sensor was calibrated. For those seeking the most accurate speed and trip values, it is recommended that an instrument supporting multi-point speed through water calibration and an instrument speed through water transducer should be included within the system. SOG and Speed will always differ due to the effects of leeway. Similarly, GPS trips and GPS logs will always differ due to the effects of leeway. In the most extreme example should a vessel be drifting in a strong current, then SOG, GPS Trips, and GPS Log will respond to the movement of the vessel. However, when so drifting, the Speed, Trip, and Log values may indicate little or no movement relative to the water, as the vessel's reported speed relative to the water (i.e. current) would be zero.

Please note that log/trip will generally differ from GPS log / GPS Trip due to affects of current. Under slack conditions, calibrated log / trip should be fairly close in value to GPS log / GPS Trip. Speed through water sensors feature non-linear error. As such, these sensors will be most accurate at the speed that the are calibrated at. Hi performance instruments (ex. i70, ST70, ST290) feature multi-point speed though water calibration, while fishfinders and some instruments (ex. i40 Speed/Bidata, i50 Speed/Tridata, etc.) feature single-point speed through water calibration.

With respect to resetting these log and trip distances:
- Ground Log: designed to run continuously, much like a vehicles odometer ... may be reset only by executing the System Settings Reset (HOME->SET-UP->MAINTENANCE->SYSTEM SETTINGS RESET) or System Settings and Data Reset (HOME->SET-UP->MAINTENANCE->SYSTEM SETTINGS & DATA RESET) commands ... these will reset all of the system's settings and the latter will additionally cause all waypoints, routes, and tracks stored within the MFD's memory (excluding those saved onto memory cards) to be deleted.
- Ground Trip 1 - 4: may be reset via the command sequence HOME->SET-UP->TRIP RESETS->GROUND TRIP 1-4 RESET
- Log: designed to run continuously, much like a vehicles odometer ... may be reset only by resetting the fishfinder sonar module is providing speed data to the system or by commanding the a factory reset to the instrument which is providing speed data to the system
- Trip: may be reset via the command sequence HOME->SET-UP->TRIP RESETS->TRIP RESET should a fishfinder sonar module is providing speed data to the system
or by a Trip Reset command on the instrument which is providing speed data to the system

It is recommended that one verify that the MFD's Fishfinder application settings have been properly configured to specify the model of transducer installed (HOME->FISHFINDER->MENU->SET-UP->TRANSDUCER SET-UP->TRANSDUCER) (EX. B744V/B744VL, P58/P65/P66, etc). When interfaced to a B744V, B744VL, P58, P65, or P66 transducer, the Speed Transducer configuration setting (HOME->FISHFINDER->MENU->SET-UP->TRANSDUCER SET-UP->SPEED TRANSDUCER) should be configured to "DEFAULT".

The a/c/e/eS/gS-Series MFDs support the following three options for showing Speed (through water) within your system:
- Customize the MFD's Databar to add Water Temperature
- Configure the application to display a Databox which has been configured to display Water Temperature
- Customize one of data cells within a Data application data panel to display Water Temperature.

Information concerning these data related features maybe found within the latest version of the LightHouse Multifunction Displays Operation Instructions.

The procedures below specifies how a Databox may be displayed and then populated with the Water Temp data item. The MFD supports displaying up to two databoxes with an application (ex. Chart, Fishfinder, Radar, Weather, etc.) page/pane. Within the application being viewed use the following command sequence:
1. Select MENU->PRESENTATION->OVERLAYS->DATABOXES->DATABOX 1 (or Databox 2)->ON.
2. Select MENU->PRESENTATION->OVERLAYS->DATABOXES->SELECT DATA (menu option beneath Databox 1 or Databox 2)->SPEED->SPEED

The procedures below specifies how to customize the Databar to display the Water Temp data item:
1. Select HOME->CUSTOMIZE->DATABAR SET-UP->EDIT DATABAR
2. Select the Databar cell to be modified
3. Select SPEED->SPEED from the Select Data menu

Should your system not be equipped with a Speed (through water) sensing transducer and should you desire to display SOG, then substitute "GPS->SOG" for "SPEED->SPEED" within the procedures specified above. Similarly, these databoxes may be populated with Ground Log, Ground Trip 1-4, Log, or Trip data items.

RAYFAQ#

[markpierce]

It is distance over seabed versus distance through water. Difference would be due to water currents assuming instruments are 100 percent accurate.

[Comodave]

Distance over land will be much tougher in your boat than distance over the water...

[AlaskaProf]

Same concepts in the aviation context are called "airspeed" and "groundspeed". The first is the velocity through the sustainng fluid, the latter is the distance traveled over he surface of the earth, generally the number you need for navigational calculations.

In the marine context, the only reason you would find the speed through the water column useful would be if you have a specific speed you know gives optimum fuel burn in real time. I can think of no reason you would want to store those cumulative data for the end of day or end of voyage.

[menzies]

If you do 10 knots up the river against a head current in your boat for an hour, you will have covered 10 nm over water.

If the head current affects you by 3 knots per hour, that means you will only done 7 nm over land.

So if you are headed to your favorite lunch restaurant which is 20 miles away for a 12 noon reservation, and you travel at ten knots into a three knot current, you had better be underway at 9am. That will get you there just before noon and time to tie up!

[caltexflanc]

If you do 10 knots up the river against a head current in your boat for an hour, you will have covered 10 km over water.

You meant to say nm.

I never thought to calculate distance through water, but it makes sense to me now as a more accurate way to calculate NMPG. Funny thing, I paid quite a bit of attention to STW.

The kind of running we did, by the end of the day current with and current against usually washed out during the time between fill-ups. But not always; for instance using the current up and then back down the Hudson, one of my favorite games. Or, riding the effects of the Gulfstream for long distances going north. A little wonky, but now I get it.

[Benthic2]

But if his speed and distance is GPS derived, won't it always be quoted as speed over ground ? I'm assuming he's not measuring his speed with some sort of impeller or dragged resistance device.

Wasn't this distinction only relevant when you used a table of rpm vs speed to figure your speed ? Pre-GPS if you knew 1800 rpms was 10 knots, and you went up that same river at 1800rpm, you had no way of knowing your true speed over ground...but that was a long time ago. With a GPS speed is speed.

[AlaskaProf]

Originally Posted by Benthic2

But if his speed and distance is GPS derived, won't it always be quoted as speed over ground ? I'm assuming he's not measuring his speed with some sort of impeller or dragged resistance device.

I would assume exactly the opposite. His system seems to be comparing two different sensors.

[menzies]

Originally Posted by Benthic2

But if his speed and distance is GPS derived, won't it always be quoted as speed over ground ? I'm assuming he's not measuring his speed with some sort of impeller or dragged resistance device.

I have a B&G Quad System and a Raymarine Tridata that both give speed through the water and three GPS systems (Furuno Navigator, Garmin 4208 and 3210) that give speed over the earth.

Frankly I never look at the B&G and Tridata speed, just the GPS. The only reason I see to do so would be to see impact of current (like Gulf Stream) on my speed. But not sure how I would use that info.

[psneeld]

its simple, dont complicate it.

after a lifetime of navigating, not sure why distance though water is relevant.

Engine hours seems to cover the important difference.

[Bacchus]

I think its easier to understand the difference between Speed (compared to the water using an impeller or similar device) and SOG (speed vs the bottom measured by a GPS)

If you understand that difference then distance is simply Speed X Time.

[psneeld]

Unless sailing (even then weak), trolling lures, or figuring out bottom paint wear.....

I see no need for speed or distance through water.......

[cafesport]

It’s been a while since I’ve messed with a ray marine plotter but as I recall what you want is to use the ground trip. Reset three of the four trip logs before you leave. Reset trip one daily and perhaps 2 weekly. Trip 4 will show boats total distance trip 3 will show distance since departure and other two will show what you want based on when you do the reset. The differences between sog and stw are important for collision avoidance calculations. Sog being ground stabilized and stw being sea stabilized and is one of the reasons ais plots can vary from radar plots.

[RCook]

I find Speed Through Water interesting, as it tells me about the current if I compare to SOG. Quite interesting in the PNW. So I would fiddle around with calibration of STW until it matched SOG in zero current.

One interesting thing on my 26-footer was that accuracy of calibrated STW would vary considerably depending on how fast I was going. With a planing hull and transom-mounted speed wheel, going faster or slower would change the water flow around the speed wheel. Calibration would be WAY off at planing speed if it was about right at 6 knots, and vice-versa. At slow speeds, even a knot more or less made a notable difference.

Eventually I figured out that I wanted STW to be accurate at my most common slow cruising speed, and calibrated it for that.

For monitoring how many NM we travel, I let the chartplotter compute that based on SOG.

[Alaskan Sea-Duction]

Thank you everyone

[psneeld]

Originally Posted by cafesport

It’s been a while since I’ve messed with a ray marine plotter but as I recall what you want is to use the ground trip. Reset three of the four trip logs before you leave. Reset trip one daily and perhaps 2 weekly. Trip 4 will show boats total distance trip 3 will show distance since departure and other two will show what you want based on when you do the reset. The differences between sog and stw are important for collision avoidance calculations. Sog being ground stabilized and stw being sea stabilized and is one of the reasons ais plots can vary from radar plots.

please explain how the difference between the 2 are for collision avoidance?

at least how stw is important....

[dhays]

Originally Posted by psneeld

Unless sailing (even then weak), trolling lures, or figuring out bottom paint wear.....

I see no need for speed or distance through water.......

After sailing for 50 years in an area with lots of tidal current, I always want to know boat speed.

My current boat doesn't have any boat speed indicator, only GPS SOG. It honestly drives me crazy. Just as I wanted to see boat speed when trimming my sales, I would like to see boat speed when adjusting trim tabs or figuring out the best rpm/speed/distance combination. The current here, even at supposedly "slack" tide and change significantly in the space of 20 yards.

I understand the reasons and reasoning behind the POs decision to not add boat speed. Even Trevor from North Pacific feels that it simply isn't needed and he grew up boating in BC and is a pilot, so he understands the vectors involved. However, I think someday I will likely add boat speed to my boat.

[BandB]

Originally Posted by caltexflanc

You meant to say nm.

I never thought to calculate distance through water, but it makes sense to me now as a more accurate way to calculate NMPG. F

That's why all performance and speed testing consists of two runs in opposite directions, running from Point A to Point B and back and they average the speeds on the runs. Typically very little difference but always some.