Cartographer: Scanned maps

wrote by Don Maxwell, many thanks to his tribute

I've been experimenting with scanning sectional charts to use with Atlas. I've tried a lot of different scan resolutions, trying to figure out which is the best resolution and how much of a chart is best to scan.

Black and white scanning at 100 dots per inch seems to be the most practical scan resolution to use. If you decide to begin collecting sectional chart submissions from Atlas owners, this might be the best scan resolution to use as a standard. Here's why I think so:
 

Color

I've tried scanning in color and gray scale, but both require a lot of work to produce a useable two-color bitmap. (I don't have a Palm III, so I can't test gray scale maps in Atlas.) Black and white scans are simple, and they seem to work reasonably well on sectional charts. I'm using a Microtek Scanmaker E6 and PhotoImpact software.
 

Resolution

The Palm display is 160 x 160 pixels, and it's about 2 3/16 inches wide and high. (That's about 56 mm, but my scanner software uses dots per inch.) That works out to approximately 73 pixels per inch in the display, but scanning at that resolution is too coarse for small text to be readable.

Scanning at 146 or 147 dpi double the Palm Pilot display resolution produces very nice results, but the image is larger than necessary for legibility, and of course it consumes more Palm memory. At 100 dpi, however, most text is legible and most map features show up reasonably well. (Scanning at 110 dpi is 150% of 73, which also reduces pixelization, but it isn't significantly better than 100 dpi.) Furthermore, 100 dpi is convenient in Atlas, because the scale of the map is about 11 nautical miles, or about 20 km, across the Palm screen easy numbers to remember.
 

Example

Here's a 300 dpi scan of my Palm Pilot Pro displaying an Atlas view of the Petersburg, Virginia, airport.

As (I hope) you can see, it's not a really sharp image, but most of the text is legible, and most landscape features are recognizable, including the magenta class E airspace around the airport. If you look closely, you can even read the CTAF and AWOS frequencies.

The scale is about 150% of the actual sectional chart. That's bigger than I'd like, but so far I haven't been able to produce bitmaps below 100 dpi that are useful in the Palm Pilot.

Here's the actual bitmap of that portion of the map (you should see it doubled in size here):

The entire map in this scan covers 2 degrees of longitude and 2.9 degrees of latitude. It's 648x773 pixels and 63,289 bytes--and it fits into Cartographer and Atlas! (I have a TRG 2+2 meg RAM card.) I made it so large because it covers the entire area surrounding the airport from which I normally fly.

Scan area

The projection in sectional charts is such that the lines of latitude are parallel, but the lines of longitude are not. Therefore, the larger the area scanned, the greater the location error there will be in Atlas. For that reason, I think it's best to scan no more than one or two sectional chart grid rectangles at a time. This keeps the error to a minimum. I've found that scanning two horizontal grid rectangles is a good compromise--at least at the latitude where I live, in Virginia. The rectangles are higher than they are wide, so two side by side makes a map that is more or less square and is a convenient size for Cartographer and Atlas. Side by side rectangle-pairs contain less error than vertical rectangle-pairs do.

However, scanning one grid rectangle at a time (at this latitude) produces even smaller errors, especially at higher latitudes. Also, individual grid rectangles might be easier to keep track of in a database. They could be identified by latitude and longitude numbers, such as 37.0&37.5, -77.0&-77.5 or some similar scheme.
 

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