Networking 3rd Party GPS/GNSS into Lowrance

Ray Valley

Aquatic Biologist and BioBase Product Expert

I frequently get inquiries from current and prospective BioBase users about the accuracy of consumer-grade Lowrance GPS and whether survey-grade 3rd party receivers capable of differential correction (DGPS) or receiving positions from multiple satellite constellations (Global Navigation Satellite System – GNSS) could be used with Lowrance and processed with BioBase.

The first question about accuracy prompted a test in March of 2013 with a Lowrance HDS tested side-by-side with a Trimble GeoXH.  I was pleased to find less than 1m deviation on average from post-processed Trimble DGPS positions.  One meter accuracy and precision is typically sufficient for most boat-based mapping applications. Still, prerequisites for some projects require DGPS, and there are a number of BioBase users who have and still would prefer to have DGPS generated positions to use when logging trips. Thus, I was interested in exploring the capabilities of networking positions from a third-party receiver into a Lowrance HDS.

Continue reading “Networking 3rd Party GPS/GNSS into Lowrance”

Aquatic Mapping Strategies

Producing professional-quality aquatic maps has never been easier with Lowrance and BioBase mapping technologies, but there are several strategies that can help you optimize your time on the water and produce the best possible map output:


Make Sure Your Transducer is Aligned Correctly
An angled transducer is the most common oversight of users and has been the subject of blogs in 2013 and more recently in 2014 (Figure 1).

Figure 1. Example of a misaligned skimmer transducer and effects on the sonar signal.  For good BioBase outputs, your Lowrance transducer should be aligned parallel with the ground (inset).

A misaligned transducer will result in weak return signals over flat bottoms or slopes where the beam is pointed away from the slope (Figure 1).  In contrast, if the beam is pointed toward the slope, the signal return will be much stronger than normal (Figure 2).  A misaligned transducer can result inaccurate bathymetry, aquatic vegetation, and bottom hardness outputs.

Figure 2.  Example BioBase output where a transom mounted transducer was slanted downward.  In this case, with the boat moving from SW to NE along shore, signal strength is much more diminished on the downslope than on the upslope where the signal is more direct.

Make Sure Your GPS is Aligned With Your Transducer
A second common oversight is large horizontal separation between the GPS receiver (X,Y position) and the transducer (Z position).  This is especially problematic on console-steer boats where the Lowrance Display with an internal GPS may be a good 5 ft (1.5 m) from the transducer.  This means the location of the mapped depth will be 5 ft off in one direction and another 5 ft off in the other direction if the surveyor is doing back-and-forth passes (Figure 3).  A misaligned GPS will result in local imprecision and a “crinkly” looking contour map, but should not affect lake-wide statistics.

Figure 3. Result of a misaligned GPS. In this instance, the surveyor was using the GPS position from the internal antenna in the Lowrance HDS display that was 7 ft away from the transducer where the depths were being recorded.  This positional bias was doubled as a result of the back and forth transect passes (red lines).

BioBase users have two “first party” external GPS antenna solutions which can be mounted right over the transducer and communicate with your Lowrance or Simrad Display.  The Point-1 GPS is a low-cost solution that suits most user needs (Figure 4).  Although the published horizontal accuracy is 5-m, users may expect much better than this, especially in open water.  In 2013, in tests with a differentially corrected Trimble GeoXH 6000, we found differences compared with the internal Lowrance HDS antenna (which uses similar technology as Point-1) to be less than 1-m.  Still, if the user requires more rigorous horizontal positional standards, they can opt for a Simrad HS60 DGPS (Figure 4).  Lowrance and Simrad also support any NMEA 2000 compatible third-party GPS receivers.  Thus with the appropriate NMEA settings and connections, users can stream positions from a wide-range of stand-alone receivers that they may already have as part of other survey work.

Figure 4. Users can align their GPS position with depths by using an external GPS antenna.  Point-1 GPS (left) is the lowest cost and most popular option.  The Simrad HS60 ensures higher reliability of 1m GPS accuracy with its capability of differential correction.  Both antenna are NMEA2000 compatible.

Design and travel transects in a way that maximizes coverage of the features you want to map
A question anyone who desires to make a quality map must address is what is the minimum amount of coverage needed.  BioBase uses the broadband, down-looking 200 kHz signal that collects “point” samples (actually bundled samples from a rapid-firing, 10-20 ping-per-second transducer) directly below your boat (see more about this here here).  Kriging interpolation is a geostatistical way that BioBase employs to predict depth, vegetation, or hardness in unsampled locations.  The more closely spaced the samples (or boat passes), the better the predictions between passes.  You need fewer passes for simple bowl-shaped shallow lakes than convoluted, deep lakes with complex bottom topography.  Same goes for homogeneous vs patchy vegetated or hard bottoms.  Here are some simple transect designs that BioBase users have found successful for creating good bottom maps:

Figure 5.  Parallel to shore design in a flooded reservoir bay.  Note the precision of the internal-GPS track which never crossed structures or shore ensuring confidence in the accuracy of resultant contours.
Figure 6.  Concentric circle transects may be the best data collection approach in small ponds or bays (the pond pictured is 3 acres; 1.2 ha).  In a small boat, kayak, or canoe, the user starts logging as they travel close to shore slowly working their way to the middle of the pond.  This design typically results in smooth bathymetric contours.
Figure 7.  Traveling parallel to the longest shoreline is an effective way of mapping large lakes (the lake pictured is 250 acres; 101 ha). Adding a single trip around shore may enhance the precision and accuracy of nearshore habitats which are often patchy. This approach can be used simultaneously with other lake sampling like aquatic plant point-intercept sampling.  
Figure 8.  Back and forth passes over an experimental plot on Lake Tohopekaliga, Florida USA.  This approach to mapping invasive aquatic plant infestations is much more efficient than using visual cues to know where the edges of aquatic plant beds occur.  We discuss this in more detail here.

Space transects according to the desired level of detail

In most cases, it’s unfeasible to completely cover the waterbody you are trying to map. But what area must we cover to produce an accurate and sufficiently precise map?  Users should be prepared to answer the following questions:
  1. What is the size of the waterbody I am trying to map?
  2. Do I want a whole lake map or just a sample area?
  3. How complex are the habitats/bottoms I am trying to map
  4. What level of detail do I need?
  5. What are the consequences of missing some detail if my transects are too wide?
  6. How much time and money do I have available to devote to the mapping project

By default, BioBase produces a 5-m by 5-m square level of detail (grid cell size) for all mapped layers and creates 5-m grid cell predictions to 25 m away from the trip path (i.e., 5 grid cells).  If the adjacent transect is less than 50-m away, the map will be complete.  If the adjacent transect is greater than this distance away, then the area > 25-m is “blanked” and no output is produced.  Users can increase both the grid cell size and buffer “fill” by increasing the buffer in the Trip Reprocessing tab in their BioBase account.  By increasing the buffer, users can “generalize” or “smooth” BioBase outputs.  In some cases smoothing will reduce local precision without sacrificing overall accuracy.  But in cases where significant bottom features are not sampled (e.g., holes, humps, points, patches), then increasing the buffer will reduce accuracy and precision.  This returns us to critically evaluating questions 4 and 5.  Below are some visual examples of how transect spacing and design can affect mapped outputs:


Try a Hybrid Approach
You can cover big water while not sacrificing detail if you employ a hybrid approach that combines wide spaced transects with closer follow up transects in areas of interest.  North Carolina State University showed us how this can be done in North Carolina’s largest natural freshwater Lake Waccamaw (8,938 ac, 3,617 ha).  The invasive aquatic plant, Hydrilla, recently invaded Waccamaw and researchers in NC State’s Aquatic Weed Science Program (led by Dr. Rob Richardson) needed to know how widespread the plant was to develop a good management plan.  They started with running 300 m transects on a couple of boats, while also collecting vegetation point samples.  This took about a week to finish the field work and a map with 60-m grid cells was processed and merged in BioBase in a matter of hours (Figure 9).

Figure 9.  300-m transects (red lines) on Lake Waccamaw overlain onto a submerged vegetation map processed by BioBase.  Red represents vegetation growth close to the surface, green is low-growing vegetation.  Blue areas have no vegetation growth.  Grid cells were 60 m.

Physical plant species samples suggested that most hydrilla growth occurred near the boat launch in the NW part of the lake which was not surprising since most new species introductions originate at boat launches.  Accordingly, NC State researchers required some more detailed information about the plant beds in this area and implemented a more intensive survey in the launch area.

Figure 10. 50-m transects (red lines) on Lake Waccamaw recorded over an area near the boat launch with relatively high hydrilla cover. Grid cells are 10-m

When we zoom in and look at the map outputs of the alternative mapping strategies we can make some informed,visual conclusions about alternative mapping strategies (Figure 11).

Figure 11. Vegetation biovolume maps of the NW corner of Lake Waccamaw processed by BioBase and converted to a raster in GIS.  The map on the left was created with 50-m transects; the map on the right was created by 300-m transects.

The first striking comparison between the 50-m transect map and 300-m transect map is the difference in the detail, patchiness, and highs and lows of vegetation Biovolume.  However, when you look at the statistics, on the whole, the percent area that has vegetation present and the overall vegetation height (expressed as avg percent biovolume) doesn’t look too much different.  This is a visual, geospatial representation of the difference between accuracy and precision.  Both are accurate results and show the same general trends.  However, the map on the left is more precise due to the higher number and closer spacing of transects than the map on the right where the map was generated from maybe one or two transects.

Now you are empowered!
This blog was meant to cover mapping strategies A to Z quickly and get you on your way to creating high quality BioBase outputs with your Lowrance or Simrad Sonar and Chartplotter.  If this brings to mind questions or creative ways you’ve navigated these issues, please comment or sign up and post a forum discussion at http://aquaticmapping.prophpbb.com/.  We are always impressed with the innovative mapping solutions of BioBase user community.  This community had presented these great examples that help newcomers to the technology get up and running quickly!

GPS Accuracy Test of Lowrance HDS


At Contour Innovations we put Lowrance HDS to the test for GPS precision and accuracy.  We know the importance of accurate maps but also recognize that “consumer-off-the-shelf” doesn’t mean it won’t provide the type of accuracy needed for accurate acoustic mapping.  The question lies more in how precisely accurate we can map aquatic environments with a “survey-grade” versus consumer GPS.  There are a lot of considerations when mapping from the surface of a water body.  Not only the accuracy of the GPS signal itself but the movement of a survey vessel on a liquid surface, wind, number of points surveyed, survey design, depth, acoustic cone degree, etc.  The list goes on because plants grow, you’re usually in a boat and water moves.  But, we can still investigate the precision of the WAAS corrected GPS from Lowrance HDS.  We were happy with our test results . . . but not surprised!

Units Tested:
  • Trimble GeoXH 6000 Series (post processing DGPS correction to 12” accuracy and precision)
  • Lowrance HDS-5 (WAAS-Correction Enabled)
  • Lowrance HDS-7 Gen2Touch (WAAS-Correction Enabled)

Methods:
  • Two individuals recording tracks while walking in same footprints, units held at chest level
  • One individual recorded a track with the Trimble Unit while the other held the HDS
  • Process repeated with the Trimble and HDS7 Touch
  • Data collected in a 2-acre soccer field in Minneapolis surrounded by trees
  • GPS Track lines from both units were uploaded to ArcGIS and converted to points
  • Point layers from both units were spatially joined and distance from each HDS track point to the nearest Trimble GPS track point was calculated
  • Conditions: Clear skies and HDOP (Horizontal Dilution of Precision) was less than 3
  • Testing Completed March 14, 2013

One glaring item that can be pulled from the chart above is the accuracy of the Trimble unit before DGPS correction.  Published accuracy is much different than actual accuracy.  You can see from the numbers above that the DGPS correction didn’t adjust the Trimble track by much. When compared against the static HDS output, the comparison hardly changes (from an average difference of .71m between the HDS7 Touch and Trimble™ DGPS before correction to .69m after correction and .45m to .83 respectively for the HDS5).  Even after DGPS correction, both HDS units performed extremely well with significantly less than 1m average difference between tracks (.69m for HDS7 Touch and .83 for the HDS5).  
At Contour Innovations we’re focused on best and uniform geostatistical models, acoustic processing, number of data points, and other key standard operating/data collection procedures to create good maps.  The average difference shown in the chart above could even be significantly less than the size of your acoustic cone (depending on cone angle and depth).  Spacing of your sample points is also very important.  The Lowrance HDS system records up to 20 pings per second.  The precision and accuracy of a map created from such voluminous data sets is unmatched.  When analyzing this much data during your survey the geostatistical model and spatial references are substantially improved.

Geo-statistical algorithms:  No acoustic map is made up of a complete data set.  Data sampling points with less that 100% coverage still require a statistical model of extrapolation or interpolating of neighborhood points.  All aquatic maps are created with some level of geo-statistical model like kriging.  Ensuring accuracy of actual points will help decrease error coefficients of estimated data but more important is the type of geo-statistical model and spacing between data sampling sights.  There is a positive correlation of error coefficient and transect spacing.  We recommend transect spacing of less than 50m and even higher resolution and lower spacing depending on mapping objectives. 

Geostatistics is a branch of statisticsfocusing on spatial datasets originally developed to predict probability distributions.  A number of simpler interpolation methods/algorithms, such as inverse distance weighting, bilinear interpolation and nearest-neighbor interpolation, were already well known before geostatistics, but it goes beyond the interpolation problem.  Kriging, the model we use, is a group of geostatistical techniques used to interpolate the value at an unobserved location from observations of its value at nearby locations.  This means that as you collect data along a transect, those data can be used to predict unobserved data between points to a statistically significant probability.  A good geostatistical model and the number of sample point are key to a complete and accuracy map! 
A couple things to consider that could influence the accuracy and precision of your maps:

  • Pitch, Roll and Yaw – Wave action or other movements of the boat as you take a physical samples
  • Tree Cover – which isn’t as common when mapping open water like lakes
  • Relation of GPS antennae to transducer – Even with 12 inch DGPS accuracy, if you’re standing 3 feet from your transducer your data points will be off.  If you take a core sample and enter the results into a GPS device, your boat could easily have drifted a lot more than your potential error.   With Lowrance HDS we provide an external antennae that can be mounted directly above your transducer so your data collection is happening at the point spot of your GPS signal.
  • Overall Survey Design – The spacing of your transects is key as it relates to the ability to confidently make predictions in unsampled locations with your geo-statistical model. 
  • Speed of Travel – When looking at a wide range of data collection techniques and methods, speed is always the biggest consideration for accuracy and coverage.

“Published Accuracy” is much different than actual accuracy.  A lot of this is a guarantee from the manufacturer to be less than a certain error threshold at least 60% of the time and is not a minimum.    
Because of this, there’s an opportunity to use scare tactics to discount the power of an off-the-shelf acoustic unit or GPS.  But, as we’ve described here, there’s a lot that goes into making a map!
We were very impressed with the performance of the WAAS corrected Lowrance™ HDS when compared against a system like the differentially corrected Trimble™ unit.  Though, we can’t say we’re surprised!
For more information on getting the best and most accuracy maps please contact one of our fisheries biologists and GIS experts.   
Lowrance™ and Trimble™ are registered trademarks of Navico, Inc. and Trimble Navigation Limited respectively.  Neither Company contributed, authorized, or requested this testing.       

Lowrance GPS Accuracy: Seeing is believing!

A quick post to demonstrate the precision of Lowrance’s internal WAAS corrected GPS antennae is in a variety of open water environments.  Docks? Boat lifts? Overhanging trees?  No problem.  WAAS correction in North America is explained here.  Have a look at a couple examples in ciBioBase:

GPS Track from a Lowrance HDS on Newport Bay, California overlain onto a bathymetry map created by automated processing of the Lowrance .sl2 log file by ciBioBase.  This trip was used for water volume calculations, bathymetry, and vegetation mapping

GPS tracks and ciBioBase derived bathymetry map in a 3-acre pond in a wooded valley in a metropolitan area of Minnesota, an example of retention pond volume monitoring.

GPS tracks and ciBioBase derived contour map of a 3-acre pond in Illinois for water
volume and aquatic vegetation analysis

GPS tracks around docks and boat lifts and ciBioBase derived contour map on Grand Lake O the Cherokees near Tulsa  Oklahoma.  The satellite even shows data collection in an area where a boat can be moored next to the dock.  That’s close!