DPlot

Perhaps this has been answered somewhere but I couldn't find what I wanted with the search query.

I have an http://www.ngdc.noaa.gov/mgg/gdas/gd_designagrid.html bathymetry data set with data points in what I believe to be a 3 second area grid. If my math is right this means a nominal grid point area of ~ (300x300) 90,000 sq. ft or ~ 2 acres. I'm wondering how many adjacent grid points are necessary for the object to be seen on a map?

Thanks,

Dave Hurd
Hilo, Hawaii

Dave,
Just so we don't confuse anybody else reading this, this:

...this means a nominal grid point area of ~ (300x300) 90,000 sq. ft or ~ 2 acres

is close to correct in your area. 3 seconds = 287.8 ft EW, 304.1 ft NS, total of 87,516 sq ft... which is still very close to 2 acres. But farther north it wouldn't be correct. For example way up north in Vicksburg MS the distance between two points at the same latitude, 3 seconds apart is ~258 ft, in NYC it is ~231 ft, in London it is ~189 ft, and in Fairbanks it is ~129 ft (less than half the distance in Hilo).

I don't know that this answers your question, but if you have a 2x2 grid of points with a different elevation than the surrounding area you should be able to pick it out on the map. That of course assumes you are careful with the contour intervals.

I'm still not sure that I'm answering the question you're asking, but my previous response needs a bit of editing:

If a rectangular object you're trying to map is 3 sec x 3 sec, then there are 3 possibilities for how it will be shown on the map: a rectangle (which you'd get only if the edges of your object all fit within a 2x2 grid of mapped points), a ridge, or a spike. With a map resolution of 3 sec, to guarantee that your rectangular object will show up as a rectangle requires that it be at least 6 sec x 6 sec - roughly 580 ft EW, 608 ft NS near Hilo.

The NOAA site is a great resource, but if you're searching for submerged objects smaller than the Pentagon you'll need better resolution. I don't have the link at my disposal at the moment, but I know there is an outfit on the east coast that has used magnetometer output to plot... actually I'm not sure... I would guess iron content, or perhaps density, and used DPlot to find a ship's anchor. Cool, or what?

Hi Dave,

Thanks for the thoughtful and detailed reply, both explaining and allowing "a little latitude" in your answers.

It's interesting that in some plots, a breakwall feature that's only 15-20' wide and about half that high above the water, stands out much better than a submerged reef that's several acres in extent with a 50' drop! WTH?

Playing with the contour intervals is always illuminating. Thanks to the NOAA set and DPlot I've been able to work with several local sports fishing guides to give them a few new places to explore.

aloha,

Dave Hurd
Hilo, Hawaii