I was wondering if anybody had any insight or thoughts on a technological bottleneck we are facing in data collection.

Basically, we have vessels (single operator) dig the substrate for collection purposes in water depths generally shallower then 35' (maximum of 35' but average about 15') via bullrake (large clam rake with a telescopic handle). The digger stands aboard a vessel with the rake and is drifting along by current/wind, thus completing a "transect" after a period of time based on how quickly the vessel is drifting. Transects typically range from 5-50m in length. Currently we are measuring the length of the transect (which is define as the point at which the rake first begins digging along the bottom until the point digging ceases on the bottom) via GPS fixes at the start and end of each transect.

At it's best, this induces inherent inaccuracy in transect length relying on GPS (although GPS accuracy is at least recorded at each fix). I was wondering if anybody here could think of a solution to record transect length more reliably or a workaround to equate transect length more accurately? Is there a piece of equipment which could be affixed to the rake to measure its distance traveled?

Proper solutions must be able to be operated by a single person while aboard a vessel and be straightforward and quick enough to rival (preferably beat out right) the time it takes to allow for a proper GPS fix and manual recording of coordinates.

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    $\begingroup$ What kind of GPS are you using? Handheld or geodetic (i.e. capable of recording raw data or real time differential corrections) $\endgroup$ Commented Jan 7, 2019 at 16:42
  • $\begingroup$ Not going to lie, this is not my forte so I don't know what qualifies a geodetic gps. That being said, we record GPS coordinates manually from each vessels fixed chartplotter (almost all Garmin's) as well automatically from the tablet's internal GPS that they record data on. $\endgroup$
    – Thomas
    Commented Jan 7, 2019 at 16:57
  • $\begingroup$ I don't know any equipment that can do this, but from a purely technical perspective something that was fully computerized, that took the readings & recorded them is what you'd need. In addition to taking GPS readings at the beginning & end of each transect taking extra reading in between would help with accuracy. $\endgroup$
    – Fred
    Commented Jan 7, 2019 at 17:04

1 Answer 1


The easiest and larger improvement in accuracy you can implement is switching from the non-differential single frequency GPS measurement to a double frequency and/or differential GPS measurements. That usually takes down the error of single measurements from a few meters to a few tens of centimetres or less.

The GPS systems that are within cell phones, tablets and most ships calculate their position standalone and in real-time using only one of the two civilean frequencies that the GPS satellites broadcast. The greatest source of error of those measurements are the discrepancy between the atmospheric and ionospheric models and the reality.

There are two workarounds to this problem, both often used simultaneously to get even better results.

The first workaround, is two use more sophisticated GPS instruments that can record the two civilian frequencies that the GPS satellites transmit. As each frequency is refracted by the atmosphere in a different way, the GPS can better calculate its effect and correct for it.

The second workaround is to do differential measurements. That is correcting the measurements of your GPS based in the errors recorded at the same time by a nearby GPS station at a fix known position. That is call a base station.

You can install your own base station on the ground near the area you work, or use one from the international network of base stations.

My recommendation, would be that you buy a double frequency GPS. If cost is a constrain, go for an used one on e-bay and you can get an excellent quality GPS for just a few hundred dollars. Then you put that GPS to record while you do your transect and it will generate a file with raw GPS measurements. Then, later, at home you upload that file (maybe after a format conversion) to a free GPS processing service and you will get back a file with the GPS track with MUCH better accuracy than your handheld GPS. This would be equivalent to use the first workaround I described and the processing I'm suggesting here is called PPP (Precise Point Positioning).

When you record the raw GPS data and do the processing later is called post-processing, and it is the best practice if you don't need the high accuracy measurements in real-time. After a week or so, the DoD issues high accuracy corrections of the positions of the satellites and their internal clocks. So if you wait for this high accuracy corrections you can get better precision than any real-time processing. All this is already embedded in those free online processing systems. So you just need to wait a week, upload your file and wait for the result.

The next step would be to have a second GPS and set up your own base station (or check if you work reasonably close to an existing base station). However, my guess is that the gain in accuracy won't make much sense given the errors in the relative position between your GPS and your bullrake.

A second different approach would be to equip your bullrake with some sort of distance measuring wheel, that rolls over the sea floor and records the distance traveled by the bullrake. But it might be significant technical challenges to make such a device work reliably.


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