Nuts & Volts - March 2007

I suppose the best way to understand KML is to look at a Google Earth KML file. Table 1 shows a file I’ve edited for one of my near space flights. Note that I’ve shortened the number of positions in the sample file considerably. The usual number of positions listed in one of my typical KML files is over 100.

Like HTML, KML commands — or element names — are written in brackets, < and >. Parameters for a particular command are written after the <COMMAND> begins and before the </COMMAND> ends.

The first element names I edit are <DESCRIPTION> and <NAME>. When the image of a flight path is clicked on a Google Earth map, the text entered in these two elements is displayed. I like to write the mission name and date in these elements.

Next, I edit the element names under <LOOKAT>. These seven elements indicate where in space to place and point your eyeball. The <LATITUDE>, <LONGITUDE>, and <ALTITUDE> element names indicate the point above the earth to stare at. The

<RANGE> and <TILT> ■ FIGURE 2 element names indicate the distance and angle to look from. The <HEADING> element name is the compass direction your eyeball points. Since the near spacecraft’s GPS altitude is with respect to mean sea level, the <ALTITUDEMODE> is set to ABSOLUTE. Figure 2 shows my diagram of what the <LOOKAT> element names mean.

The <LOOKAT> point in the sample KML file is a location at the center of the near spacecraft’s flight ground track and half way up to its maximum altitude. Its

■ FIGURE 3. The extruded flight path.

tilt is 90 degrees (horizontal), with a heading of 0 degrees (true north). The range is equal to the flight path’s maximum altitude (I sometimes have to edit this after looking at the results). A word of caution here. The altitude and range elements are in units of meters. Until I realized that, my flight paths looked awfully tall and skinny (just like me when I was younger).

The only name elements modified under <STYLE> are

<LINESTYLE COLOR> and <WIDTH>. The <LINESTYLE COLOR> element is an eight byte field. The first two bytes indicate the transparency of the line and the remaining six bytes indicate its color. A transparency of hex 00 creates an invisible line and a hex value of FF creates a fully opaque one. The next three pairs of bytes are for the colors blue, green, and

red (in that order). In the sample KML file, the <LINESTYLE COLOR> is set to ff000000. That draws the flight path as a fully opaque black line. The <WIDTH> is set to two just because that seems to work well.

The last name element modified is <EXTRUDE>. Extruding a flight path drops a curtain from the flight path to the ground. I find the curtain confusing, so I don’t have it drawn. To NOT extrude a flight path, type,