Frequently Asked Questions -- Maps

FAQ Master Index

Where can I find some digital topo maps to use with GPSy?

The best place to start would be the Map Resource Library of this web site. The Map Resource Library has links to freely downloadable digital raster graphic (DRG) topo maps as well as ordering information for DRGs on CD-ROM. Links to other map formats are also at that location.

Then also try the map links of the GPS Resource Library. This is a more general non-GPSy specific link list.

Do you have any nautical charts or international maps?

Please try the USGS topo maps in the Map Resource Library first. Many boat users have successfully used digital topo maps to navigate in waters near land. Since many digital topo maps are free, this is a good solution for these people who don't have a lot of money.

However, if you're an open-ocean person or have money to spend, we have implemented NOAA/BSB ChartKit chart support in GPSy Pro, our enhanced version of GPSy. This will allow you to use these high quality pre-calibrated commercial nautical maps of Canadian and U.S. territorial waters. For more information, see our GPSy Pro Information Page as well as BSB information links in the Map Resource Library.

We currently do not know of many sources of calibrated map data for countries other than the U.S. We would suggest searching the internet, especially the map resource page as well as the Map Links in the GPS Resource Library.

How do I get GPSy to autocalibrate my TIFF DRG topo map?

GPSy recognizes TIFF DRG topo map files that are in any of the file formats that GPSy supports (TIFF, JPEG, PICT, BMP, GIF, etc.). Many USGS topo maps also embed geographical calibration data in "GeoTIFF" tags. GPSy is able to extract these GeoTIFF tags and use them.

If there are no GeoTIFF tags and the map file is in TIFF format with a file suffix of .TIF, then GPSy will also look for a TIFF World file with the same file name, but a file suffix of .TFW. GPSy can use the TIFF world file to geocalibrate the map. The best way to find out if GPSy can handle your map/calibration type is to try opening the map in GPSy.

Unfortunately, the TIFF world files only provide a subset of the information GPSy needs to calibrate the map. In particular, the TIFF World file only embeds the UTM coordinates of the first pixel in the upper left corner of the map, but doesn't tell GPSy what the geodetic datum or UTM zone of the map is. The datum problem is fixable (GPSy will default to the current primary display datum), but without the zone information, GPSy cannot calibrate the map to world map coordinates.

Other DRG map programs for the PC take the simple way out and never bother to correlate the map coordinates to global coordinates. But since GPSy is designed to work in the world coordinate system (the GPS system), it needs the UTM zone number.

There are two solutions. If the original TIFF/TFW file came from a USGS DRG CD-ROM, then the approximate coordinates of the map are embedded in the file name itself (ie, file C40072A1.TIF has a lower-left coordinate of 40 degrees North, 72 degrees West). GPSy can then read the USGS DRG TIFFs right off the CD-ROM using either GeoTIFF or TFW tags. Try opening your TIF file in GPSy, if GPSy can read the calibration data automatically, it should open up just fine. If you are told that the file needs the TFW zone information, read on.

Adding the Zone and Datum Information:

For our example, we downloaded the San Francisco North .TIF and .TFW files from the Bay Area Regional Database:

First, we need to get the map UTM zone number and datum information from the map. Open the sf_north.tif file up in GPSy, click on 'Cancel' in the TFW Information Dialogue (below) since we don't know the Zone and Datum yet. The map should open without calibrations.

Scroll to the bottom left corner of the map. In the index or liner notes, it should say "1000 meter Universal Transverse Mercator Grid ticks, zone 10, shown in blue".

While we're at it, why not write down the geodetic datum as well since map calibrations are more accurate with it. The map index notes says "1927 North American datum", which is indicated in GPSy as "NAD27 CONUS" (North American Datum '27 - CONtinental US).

Let's now go back and add the UTM zone number information to the TFW file. Close and re-open the file. A popup dialog will come up asking you to enter the Zone and Datum of the map. Enter the information you just ascertained above. Click on "Save datum/zone info back to TFW File" which will let GPSy write back the editing changes and you should be on your way. The next time you open the file (unless it's on a read-only media like CD-ROM), it should be able to read in the new information automatically.

[TFW Info Panel]

As a final note, .jgw files are calibration files for JPEG images and are formatted identically to .tfw files. GPSy and GPSy Pro will search for .jgw files when opening JPEG images.

No Zone Code on Map: If you can't find the UTM zone number on your map, then try using this handy guide map to UTM zones, courtesy of Mentor Software. Note that GPSy assumes that the UTM zone is located in the Northern hemisphere if you omit the UTM zone letter (such as '10S').

Manually Editing TFW Files: If you are sadistic, or if your mom let you play with MS-DOS too much when you were a kid, you can also manually edit the TFW file. Note that the last two lines are GPSy additions to the standard ArcView TFW format:

    2.438400                [X-Scaling; meters]
    0                       [Rotation]
    0                       [Translation]
    -2.438400               [Y-Scaling; meters]
    541661.722646           [UTM Easting of 1,1]
    4192638.652790          [UTM Northing of 1,1]
    10 S (UTM)              [UTM Grid Zone or Coordinate System PCS Number]
    NAD27 CONUS             [Datum]

Important Note: Be sure to edit the TFW file in a text editor that saves files in 'TEXT' format. We recommend either SimpleText or BBEdit Light. Microsoft Word, in particular, will overwrite the file in MSWord format, which is unreadable by GPSy.

Teale / California Maps and Other State Projection Systems

The Teale / California maps on CD-ROM use a non-standard, local "Teale Albers" projection system and the NAD27 datum. If you are using a California USGS Map from the Teale Data Center, you need to let GPSy know that. Choose the "100: Teale Albers" option from the GPSy TFW info dialog. If manually editing the file, add the following:

    100 (Teale Albers)                  
    NAD27 CONUS 

There are some other non-standard projection systems that also require you to let GPSy know what they are. These include the Florida Labins Teale System used in the QOQQ and DRGS maps as well as Massachusetts and Wisconsin State Projection Systems. You should read your map liner or README files to see what projection system your map is in. When in doubt, please contact us for technical support.

Why can't I open this humongous JPEG map file?

Why is my large image cached at 4000 pixels?

There is a bug/limitation in previous version of QuickTime that prevents JPEG and other 32-bit deep images from being wider than 4096 pixels. Yes, this is a very idiotic limitation in the MacOS toolbox.

Solution - Upgrade to QT4: Fortunately, QuickTime 4 finally fixes this long-standing problem, but many customers have not yet upgraded to this release. We encourage people to upgrade to QT4 as soon as possible as it greatly enhances GPSy as well as your other QT4 apps. QT4 is a free download from Apple's QuickTime web site. Note: GPSy 3.30 or GPSy Pro 1.10 or higher required.

Stuck at QT3: Nonetheless, if you are stuck at QT3 for some reason then you must work around this limitation in Apple system software. When GPSy is faced with a 4096+ pixel wide image at 24/32-bit depth, the default behavior is that it will de-rez the image and cache it so that it is only 4000 pixels wide. This avoids the system limitation.

You can also force the color space of the map to thousands of colors by holding down the option key while opening the image (clicking on the "Open" button in the Open Dialog Box). This allows images to be up to 8192 pixels wide before the bug kicks in again. Alternately, changing your monitor bit-depth down to "Thousands of colors" will have the same effect. If your monitor is reset to 256 colors, you can have images up to 16,384 pixels wide.

How can I speed up map opening on my older machine?

Large 6-10 megabyte topo maps can take quite a while to open on old 68K PowerBooks. If buying a new PowerBook G3 isn't a feasible option, here are some tips on how to speed up map derezing:

How does GPSy's map caching work?

GPSy caches the entire map image to an off-screen buffer before displaying to the user. This allows for extraordinarily fast imaging operations. You can scroll an entire 32 megabyte GeoTIFF image in real-time on an old PowerPC 601-class machine. A G3 will give you true real-time performance, even with 100 megabyte+ images.

This caching system means, however, that in the tradeoff between memory and speed, GPSy elected to consume more memory by caching the image rather then extremely slow disk-based access. When GPSy creates the off-screen buffer (image cache), it scales it to fit within the available temporary system memory. This allows you to open a larger image than your system would normally let you. If you don't have enough memory to fit the entire image, the scaling operation reduces the quality of the image. Normally this isn't an issue if you have enough memory. Most users don't even notice the lower resolution even when the memory usage has been reduced 800%.

Since GPSy uses system temporary memory (the memory that appears under "Largest Unused Block" in the "About This Computer" panel), it behooves you to create as much available memory as possible. The offscreen cache tends to perform well with virtual memory or RamDoubler, so we suggest turning those on and quitting other applications to free up system memory.

PRO: GPSy Pro improves on GPSy's memory handling a number of ways. First, it allows the user to tune where and how memory is allocated. Secondly, it uses memory in a more optimized fashion than GPSy. Most people find that GPSy Pro is faster, uses less memory and can deliver better images than GPSy.

Furthermore, GPSy Pro allows you to refresh the screen in full resolution mode at any point. This "High Resolution Screen Refresh" is particularly helpful when working with maps > 100 megabytes that can't all fit in the memory cache. You work normally work using the cached copy and when fine details are necessary, generate a high-resolution version. GPSy Pro places the control for optimizing memory, speed, and resolution in the user's hands. Just hit 'e' at any time for the high resolution refresh.

How can I plot tracks in Microsoft Excel or Adobe Illustrator?

It's possible to export your GPS track logs to text files which you then import into Microsoft Excel or Adobe Illustrator. Basically, you should use the UTM format when downloading since UTM is a square coordinate system (units are 1 meter x 1 meter). You can also use latitude and longitude, but beware that the further North or South you go towards the poles, the smaller each degree of longitude is. At the North Pole, 1 degree of longitude approaches zero.

The only caution is that UTM exhibits strange behavior near the "zone crossings." If there is interest in having GPSy force UTM zones, please let us know.

Allory Deiss has published an excellent primer on how to export your track logs from GPSy into Adobe Illustrator. If you have any questions, please don't hesitate to ask our technical support department.

PRO: Why does GPSy Pro report an error -50 opening my BSB chart file?

Problem: There is a feature of MacOS called "MacOS Easy Open". This control panel is designed to automatically translate PC and unknown file types to their Macintosh counterparts. One of the features is Text File Translation. When enabled, this translates PC text files whose lines end with CR-LF (carriage return; line feed) combinations into Mac style text where the lines end with just CRs. Unfortunately, when this translation is done on BSB files, it corrupts both the header and data file portion, rendering the BSB chart file unreadable.

This problem also manifests itself with TFW/JGW files and other text documents that GPSy and GPSy Pro rely on as well.

Solution: Turn off the "Translate 'TEXT' Documents" feature of MacOS Easy Open.

Side-Effects: Relatively none. Most Mac applications now know how to handle PC documents natively anyway. Use BBEdit Light (a freeware application) if you have trouble opening any PC text files.

This page was last updated on May 5, 2000. We've had [N/A] hits since May 25th, 1997.