Collecting Track Log Data on your GPS Unit

Track-log data is collected on a GPS/Sonar unit and forms the basis of the 3D maps that ReefMaster generates.

General Principles

Track-logs consist of a series of track-points, each of which contains a depth and a geographic position. With enough track-points in an area, ReefMaster can generate an accurate 3D model of the sea or lake bed.

Over time, simply collecting track-log data as you fish an area can provide enough information to build up a very detailed map - especially if most of your fishing is done on the move. However it can be useful in the first instance to be more methodical in your approach to mapping, traversing a grid pattern over the area of interest to collect enough data to create a basic map of the area.

·When creating the grid-lines, try to approach each area from more than one direction, which helps with resolving features of different orientations. The example image demonstrates this, showing a single track with three sets of parallels.

·Consider the maximum interpolation distance in a Map Project when deciding how far apart to space your tracks. Although ReefMaster can interpolate depths between track-points up to 100m apart, interpolating over such a large distance can give rise to very significant inaccuracies in the final map. Unless the topography of the lake or sea bottom is very flat and featureless, grid lines should be kept as close together as is practical; somewhere between 25-50m is a reasonable starting point. In shallow water with many features, even closer tracks may be required. Consider that the average transducer, with a cone-angle of 20 degrees, covers an area that is about as wide as one third of the depth. Anything interpolated outside of this range is, to some extent, "guesswork".

Collecting Track Log Data on a Humminbird GPS Unit

Logging Sonar Data

The preferred source of Humminbird data for import into ReefMaster is logged sonar data. Sonar logs provide explicit time information for track-points which is important when tide adjusting track data.

·Ensure that you have a memory card with sufficient free space inserted into your Humminbird unit.

·On arrival at your area of interest, select the option start recording to start logging data.

·Record the data. This step might simply involve drifting and fishing for a few hours, or maybe a more systematic approach if you are mapping out a new area.

·Stop recording when you are done mapping an area.

Recorded sonar data is stored on the card in the RECORD directory. To import Humminbird a sonar log file into ReefMaster, select the RNNNNN.DAT file.

Humminbird Tracks

Humminbird units save depth information as part of the breadcrumb trail that is continuously produced when the unit is in use. This means that the unit is always generating usable track-log data.

Depth data in the Humminbird current track

Many older Humminbird units have a bug whereby they stop recording depth data once the current track has filled. The current track is the track that is currently being recorded, and has a limit of some 21000 track-points. Once this limit has been exceeded, the track begins to get overwritten from the beginning and, in units that exhibit this bug, all depth data is lost. The simple workaround to this problem is to ensure that you start a fresh track for every new track-log by clearing the current track, and to never exceed the maximum number of track-points in a single track (with even the minimum 1 second track-point interval set, this still allows for nearly 6 hours of data per track).

Track-point Time Interval and Minimum Distance

The time interval between saved track-points is configurable on the GPS unit, down to a minimum of 1 second. A minimum distance is also applied, which means that a new track-point will not be saved if the vessel has not moved more than this minimum distance since the last saved track-point. Set the minimum distance to the smallest setting, to collect the largest amount of data. When setting the track-point time spacing, it is important to remember that ReefMaster calculates the time of Humminbird generated track-points using the track-point time interval. The time is important when applying water height offsets in tidal waterways. If the track-point time interval is set low (eg, 1 second), and the vessel is moving slowly (for example, drifting and fishing), the Humminbird unit may fail to write a track-point because the vessel has not moved more than the minimum specified distance from the last track-point within the time interval. The calculated time of track-points will become increasingly less accurate, the more track points are missed in this way.

Logging track data

·Check your device settings for Trackpoint Interval and Track Min Distance, which can be found in the Navigation Menu Tab*.

·Set Track Min Distance to the lowest setting. A lower track point interval will give you more data, but remember that track points are not saved unless the Track Min Distance has also been moved. Bear in mind the speed that you will be traversing the area of interest and try to set the track point interval such that you will have moved the minimum distance during that period of time. This minimises the drift in time track point time calculations, which makes for more accurate tide adjustment. For example, if you are planning to drift and fish in light winds, use a higher trackpoint interval than you would if trolling at a steady 5 knots.

·Clear the current track on arrival at your area of interest. If you want to retain data in the current track, Save the current track before clearing it.

·Note the start time of the track, or take a waypoint. Taking a waypoint to record the start time of the track can be useful to confirm that the track start time that ReefMaster calculates using the Trackpoint Interval is accurate. The track start time can then be overridden in ReefMaster if required.

·Record the data. This step might simply involve drifting and fishing for a few hours, or maybe a more systematic approach if you are mapping out a new area.

·Once you have finished recording data, save the current track.

·Record multiple, short tracks. If recording for a long period of time in tidal waters, consider saving the data as multiple, shorter tracks - say, one per hour or so. This helps minimize timing errors for tide adjustment. Simply save the current track, then clear the current track and carry on recording.

·Transfer the data. At the end of the day, transfer the data onto a memory card so that it can be uploaded to ReefMaster. Choose the option Export All Nav Data from the Navigation Menu Tab*.

* The precise names and menu locations of settings varies by unit. Consult the documentation that came with your Humminbird device.

Collecting Track Log Data on a Lowrance GPS Unit

Lowrance units do not save depth information in their breadcrumb trails, so depth information has to be extracted from saved sonar log files.

Most Lowrance units can record sonar log files in a choice of two different file formats; SLG and SL2. The SL2 file format is the more modern format, and has the capacity to record more than one channel of sonar simultaneously. SL2 files are the format of choice when collecting sonar log data for use in ReefMaster.

Track Start Time

If recording data in tidal waters, it is important that track points are assigned the correct time.Time information for individual track points is stored as an offset from the beginning of the recording. For this reason, it is important that the start time of the track is correct. ReefMaster uses the last modified time of the sonar log file as the end time of the track, and calculates the start time by subtracting the sum of the track point time offsets. It can be useful to note the start time of each sonar log, so that the start time of the track can be overridden in ReefMaster if required.

·On arrival at your area of interest, select the option start recording sonar to start logging data.

·Note the start time of the track, or take a waypoint. Taking a waypoint to record the start time of the track can be useful to confirm that the track start time that ReefMaster calculates using the individual track point offsets is accurate. The track start time can then be overridden in ReefMaster if required.

·Record the data. This step might simply involve drifting and fishing for a few hours, or maybe a more systematic approach if you are mapping out a new area.

·Stop recording when you are done mapping an area.

·The sonar log data can be read from the memory card by ReefMaster. To transfer waypoints, follow the manufacturers instructions for transfer to a memory card. Use only USR versions 2 or 3, or GPX as file format options when transferring waypoints for use in ReefMaster.

·Do not use the Low Quality setting for SLG files if you wish to view the track using the sonar viewer, or for bottom composition.

Logging Live Data

ReefMaster can create tracks in real-time by logging position and depth data via an NMEA 0183 data connection. Many units from a wide range of different manufacturers are able to output the required NMEA data, opening up a wide range of possible sources for ReefMaster data. See Live Data.