javAPRS and TIGER Maps

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Many people are aware of the TIGER map generator available on the internet courtesy of the Census Department. javAPRS now has the ability to access this server, and put APRS data on the maps. Here is an example:

There is a problem with using these maps. javAPRS assumes a linear projection, where each pixel covers the same latitude and longitude regardless of where on the map it is. This makes it much faster to plot points, at the expense of some distortion, especially on wide area maps. TIGER uses a different projection, which results in some errors in icon placements. These errors are minimal on small area (city-wide) maps, but become huge in wider (state and country) maps.

Here are two sample maps of the same area. Everyone proabably is able to visualize the "Four Corners" area of the southwest, Arizona, New Mexico, Colorado, and Utah. These borders run true north/south and east/west. The first map here is made from the DeLorme CD ROM. Notice the 4 corners area, the lines are vertical and horizontal. This is a linear projection.

Here is the same area as depicted by the TIGER system. Notice the north/south line is now a curve. This is a result of the sinusoidal projection. It looks like it would to someone in the space shuttle; realistic, but it complicates converting between x,y coords and lat/lon. The code supplied by TIGER to accomplish this is 10 pages long. My code for linear projection is one line!

Mac/Win/javAPRS all use this linear projection. People have sworn to me that APRSdos uses sinusoidal projection. Could someone display the same approximate area on APRSdos, and tell me if the line is vertical?

Why isn't there a live javAPRS applet here?

Java is a language designed for networking applications. One of the principle concerns is security. Java's built in security manager prevents someone from putting an applet on a page that erases your hard disk, sends files from your hard disk to another computer, or other such anti-social acts. Unfortunately, it also gets in the way of some cool programming. It prevents an applet from accessing any computer other than the one it was loaded from. So if I could put javAPRS up on the TIGER computer, the map display would work fine.

I can't do that. There are two options...the first is to disable the security manager. Netscape does not have this option, but (I'm told) Internet Explorer does. Also, the Sun Java Development Kit (JDK) includes an applet runner that can have the security manager disabled, and also allows you to run applets from your disk, greatly speeding up access. The JDK is free, and available from SUN for most platforms.

The other option is to have the same server that you download the applet from relay the request to the TIGER computer, and return the GIF file to you. The security manager has no way of knowing where the map originally came from, so it is happy. Steve Boyle, KD6WXD, has added this capability to his TNC data server. It is in alpha test now, and as soon as it is open for general use we'll let you know.

In the mean time, if you want to use it with the JDK, here is a link to get the javAPRS zip file. Unzip the file, and run the tigerTest.html file with the Applet Runner. You can modify this file to display any data you like, and to set up a different initial map. The parameters in the applet call sould be pretty self-explanatory.

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