map resolution

[hilo_kawika]

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

[DPlotAdmin]

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.

[DPlotAdmin]

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?

[hilo_kawika]

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