Thursday, April 4, 2013

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 statistics focusing 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.       

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