[Coral-List] Mapping procedure
kr4ah at empery.sssgroup.net
Fri Aug 8 08:54:59 EDT 2003
On Fri, 8 Aug 2003, wahszel wrote:
> Greetings! I have been mapping shallow and patch reefs in Malaysia and
> Borneo using the RoxAnn Seabed Classification System since 1996. I started
> with general habitat mapping; and managed work my way to discriminate the
> reefs down to growth form types. I am also doing the accuracy assessment
> for such mapping method.
I suppose I should list my experiences here also. I have done
survey work in small boats (3 meters in length) and large ships (150
meters in length). I have used ARGO, LORAN, Lorac, GPS, Transit
Satellites, SINS, Miniranger, Theodolites and a host of other methods that
range from very high tech to extremely low tech. I have worked locations
from the Bearing Sea through the Gulf of Mexico and a very little into the
Atlantic. I am giving this background so that readers can assess my
thoughts in light of my experience and make their own judgements.
> From my experience, it is hard to maneuver the boat and produce 'good'
> transect lines if I use a boat smaller than 20ft.
I have found that it is usually easier to precisely manouver a
smaller boat than a larger one. Think of it this way, if you are trying
to approach a buoy in the water to pick up a line, which is easier to use,
a small boat or a larger one? For me, the answer is the smaller one.
The difference between surveying and picking up a line is guiding
yourself to the survey point and what you do when you get there. The
probelm of guiding yourself to a "non-existant" buoy might be harder with
fewer instruments and therefore might be tougher is a smaller boat. The
issue of recording data depends on the characteristics of the instrument
package to be used.
For running arrow straight lines in shallow water near shore I can
think of few boats I'd rather have than a human powered Polynesian canoe.
The canoe is shallow draft and can run in extremely thin water. You can
manouver it through an opening only inches wider than the canoe with
repeatable accuracy. If it can carry the instrument package safely and
the canoe provides the necessary waterproofing, power and weight carrying
ability needed, it can be a perfect platform for recording data.
The issue then seems to be whether "arrow" straight lines are
necessary and whether they can be achieved. Straight lines do look good
on the final data sheets, but are they better than "snake-like" transects?
Given suffucient data density, it may not matter. For instance, if your
straight lines miss the top of coral heads, might it be better to ask the
"surveyor" to run the coral heads and get their depths? In my mind it
depends on the intent of the survey. It's the old Modifiable Area Unit
All that said, how does one run arrow straight lines in a small,
hand powered craft with *no* navigation instruments aboard? The answer is
to use natural or artificial range markers. Depending on the specifics of
the range line, the survey party can stay within a meter of a preplotted
survey line. At this point, I don't have the exact URL, but _The American
Practical Navigator_ (Bowditch) is available on-line in PDF format and has
fair information on range lines in Chapter 5 sec. 502 on pg 64.
If anyone needs more information, write me.
> Not only the wind and
> wave would caused a snake-like transect line; the pitch and roll will also
> affect the accuracy of the sonar readings. Traveling speed is very crucial
> too to ensure good quality data.
If I have the right kind of vessel in mind, a Polynesian canoe is
an outrigger or a catamaran and should have less roll than a monohull.
The narrowest beam and mode of transducer for the SeaCharter is 12 degrees
and I doubt a multihull or outrigger is going to heel 6 or more degrees
during normal survey activities.
As for travelling speed, I'd think you would get better
measurements going slower with less disturbance of the surface water. If
it were me trying to get a critical point reading, I think I'd prefer to
sit over the spot for a while and take a series of measurements. On a
practical basis, though, I don't think the difference of speeds involved
would make much difference in the final product.
All in all, I like the Polynesian canoe.
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