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BC Cartographic frequently asked questions


Frequently Asked Questions

Why are the image files so large ?

There are 2 issues regarding size of an image. The first is the compressed or packed size, The other is their uncompressed size (size in memory).

Image size is determined by chart width x scan resolution x chart height x scan resolution x color depth / 8 bits (byte)

Lets assume a 24" x 36" chart scanned at 240 dpi and saved as a 4-bit image (16 colors)
(24)(240)(36)(240)(4)/8 = 24,883,200 = almost a 25 mb uncompressed image.

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Why the "NOT TO BE USED FOR NAVIGATION" statement ?

The quick answer is "to protect ourselves from lawsuits". In addition to that, we feel that even the current Government produced charts which can be used for navigation - assuming they have been updated using "local notice to mariners"- are only a guide to the hazards found. Survey methods are not absolute and by the time the data gets transferred to plate for printing we then need to worry about paper stretch and alignment errors. Most mariners understand that even the best chart is not 100% accurate and neither are the digital versions.

If 254 dpi is "best" why the 240 dpi images ?

After about a year of trying to use lower resolution images yet retain functionality we gave up and started from scratch. We determined that 240 dpi was the resolution that would display sufficient information. Once we read the reasons NOAA used 254 we decided to defer to their research and started using 254 as our new baseline for future images.


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I can't load an image

There are a few reasons why this might happen.

#1) Insufficient resources in your computer - if the status bar on your image app shows a quick load and then it "stops" and you give up in frustration, you probably need more RAM to view the larger images. You might try converting to another file format.

Example: on a 24mb 486, after 2/3 of the load of a 45mb BMP RLE image it would "hang". By converting to TIF format using PaintShop Pro's batch conversion (it doesn't open the image to convert it) the image was then able to load. More importantly, the progress of the TIF load was displayed, and while it was SLOWWWWWW you didn't give up. Whereas the BMP would quickly show load status to 2/3 then appear to stop. If you waited for it, it would eventually load.

It has to do with the way the files are internally coded to be read and the ability of the application to properly manage the large file sizes.
You might consider cutting the chart into a smaller section that covers only the area you want to view if you are limited in compute resources.

#2) Your image viewer is not robust enough - applications like Microsoft Paint are not meant to be much more than entry level image processors. Many viewers are available for demo, and there are also some freeware viewers. We use PaintShop Pro for most of our image processing and it is available from JASC Inc
A pretty good viewer is ACDSee 32 ($40 US) with a 30 day shareware demo http://www.acdsystems.com

#3) A corrupt download - a lot of glitches can occur during a data transmission. Usually it results in a file unreadable by a viewer. Best advice would be to try again at a time when the net traffic might be less.

#4) A corrupt file in our directory - we do our best to make sure that the files actually work. We do this by downloading the file from our site after an upload. Misteaks :) happen and if you suspect a file is corrupt at the source, please ask us to verify it's integrity.

#5) If you to start your viewer application by double clicking on the image file name, the image application can start and the image appears to be loading BUT just as the image is loaded there is an error message reported. If this occurs, try starting your viewer application and open the image directly from the image application. The cause is probably a registered file association problem (operating system issue).

I can't print an image or a section of an image

Due to the memory and/or disk requirements of any large raster image, you may not be able to select a section of a chart and easily print it on a handy 8.5 x 11 sheet, without "running out of memory". This cryptic error can indicate a lack of sufficient swap file, temporary file space , disk space, ram or "none of the above".

Generally speaking, you will require space (ram or disk) for the uncompressed image, plus another copy for any undo file of your viewer/editor and your printer might require another copy. That usually stresses most systems and many applications don't seem to have been designed with such an expectation.

So how do we deal with this irritating problem ?
I have been able to get most applications to print a section by first selecting the area I wish to print, then "Edit - Copy" (usually Ctrl C on the keyboard). Then "Paste as New Image". You can then probably print this smaller section without difficulty.

Be aware, it might be an application issue or an Operating System issue, so installing additional ram might not solve the problem, so try the cut / paste work around first.

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