Friday, March 28, 2008

Labels via Overlays

Stefan's post about the Wilkins Ice Shelf breakup has brought several thoughts together that I will post about shortly. Today I'll discuss labels which develops ideas I have posted about before:

In his post Stefan links to a Google Earth file where he has just put two images together as overlays on the ice shelf (no criticism of Stefan is meant by this disucssion, he has just reacted to events with a quick kmz which was worth doing), you'll notice that the labels are incorporated into the images which means you can't turn them off as a separate layer. Being able to toggle labels on or off is useful as it enables you to unclutter the screen. To add a label as a separate layer you could add a path (the line) with a placemark with no icon (the label) but the problem with this is that the text will rotate around and keep its size as you zoom around. Another approach is to add the label as an overlay, I've done both in an example below:

Screen shot showing two types of way to add labels in Google Earth

example kmz file

Backstory: In the area shown the Brahmaputra is eroding paddy fields, you can see the evidence of this by roads no longer leading anywhere. I'm working on this as a lesson plan for schools.

If you zoom in and out and rotate in GE you can see the different behaviors. The placemark/path is easier to do but if you have several of them in a view the way the text keeps its size clutters the view so I think the overlay works better. Notice I also chose white as the colour for the labels as the background is dark, if your label is sitting over a mixed background of light and dark you may want to put some sort of semi transparent background beneath the label to make it stand out more.

HowTo: To produce the overlay you want to use your favourite image package (I use Fireworks) to produce text and a line as a transparent .gif file using index transparency. Valery has produced a web app that does this for you, I haven't tested it as I'm happy with my fireworks to .gif workflow but it's worth a play.

HowTo make a semi transparent background: To produce a semi transparent background I would produce a white, opaque square as a .gif and load it into Google Earth so it is below the label. You can alter its transparency in properties and make it plot underneath your label by using draw order: properties > location tab > draw order.

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