Thursday, July 17, 2008

DVD of Data Shipped to the Field

Earlier this month I posted about regions. One of the uses I noted for this feature of Google Earth was that Aid workers could be sent into the field with a DVD of data in a region. This would mean they would not need to be connected to the web to use Google Earth so long as they were only using it to view a small region (e.g. around a refugee camp).

To illustrate the point a bit further I've recorded a video from the region covered by Crisis in Dafur, click the image to view:

Via Ogle Earth I found this post from Johnathon Thompson of Aid Worker Daily discussing how Google Earth could be used in the field with a poor web connection. His suggestion was that Google should let countries or areas beyond where he is interested in be blanked out, this would reduce the bandwidth needed to view the areas he is interested in. Unfortunatly the idea doesn't work, blanking out countries only saves bandwidth when you are viewing areas at the highest altitude, there will still be a considerable need for bandwidth as you zoom in.

Explanation of the Image Pyramid: To understand this you need to understand the 'image pyramid' arrangement of how Google Earth (and all other eMaps/Virtual Globes) loads in data . Lets follow what happens as you fly closer to the ground in GE*:
  1. You start by flying to a postion above Myanmar where the on screen view you see represents 1000 km x 1000km. GE loads in image A to render the view, this image is 1000 x 1000 pixels (pixels = dots that make up the screen) in size. You are viewing an image where 1 pixel represent 1 km on the ground.
  2. Zoom in to view a 500km x 500km wide sub section of Myanmar and GE loads in a new image (B) to render the view, image A is unloaded. Image B is also 1000 x 1000 pixel in size. It differs from image A because one pixel of image 2 is 1/2km or 500m wide. As a result the on screen view is more detailed.
  3. Fly in further so the on screen view is 250km wide and GE unloads image B and loads in image C: it is also 1000x1000 pixels in size and in this even higher detail image 1 pixel represents 250m on the ground.
  4. Pan across to view the land area to the East of image C and GE loads in another 1000x1000 pixel image (D) to render the new region. Because you have flown no higher or lower the resolution of D = that of image C.
If you consider Johnathon's idea it would only impact the first view when we were looking at image A. Lets say we save downloading half the pixels of image A. However, as soon as we flew down into the country we would lose sight of all the other countries surrounding Myanmar so all the pixels making up images B, C and D, would still need to be downloaded in full. Note that all the images are the same size so image A only represents 25% of the total download GE needs, so applying Johnathon's idea we only avoid downloading 12.5% of the pixels.

Johnathon goes on to say:
"I have a heard a number of times that caching imagery is a viable solution to the bandwidth problem but the reality is that most folks have no idea how to cache imagery" (emphasis is mine)
Help is at Hand: Well that's OK Johnathon, data on a DVD can be easily arranged so that all the user in the field needs to be capable of is opening a Google Earth (KMZ) file. All you need is a techie like me to arrange it for you, DVDs with the KMZ could be copied multiple times and cached data spread to whoever needed it. With some programming work, the process could even be automated so that some non-techie on a good, cheap web connection could produce a DVD to be sent to the field pretty much at the click of a button.

He also says that:
"when the fabled ‘DVD full of imagery that gets shipped to the field’ solution comes up it takes about 30 seconds to convince the other person that it is really just a nice idea that has little basis in reality" (emphasis is mine)
Am I missing something? I'm not sure if that just refers to Aid Workers not being techies and not knowing how to cache data or if there is some other barrier to this idea I've missed? I've worked in Aid situations so I thoroughly understand that there can be no expectation of specialist GIS skills in Aid workers.

I am interested in pursuing this idea further, if anyone has a case study area they'd like to volunteer and ideas about funding I'd be happy to collaborate.

*This is a very simplified version of what actually happens in GE and I've made the numbers simple to make the point but it is broadly the way GE works.


MattFox said...

You're of course correct. Turning off other countries would save little or no bandwidth. Creating a DVD of the imagery isn't possible either unless you're willing to pay for the imagery. (about $3-$5 per square mile unless you could get it at discount rate). Purchasing imagery for an entire country would probably be too expensive.

But....Global mapper allows you to buy imagery directly from Digital Globe and you only pay for the areas you need. So you could conceivably purchase small islands of high resolution imagery near areas of interest (towns, villages, along roads, water sources, etc.). Everything else would be blacked out (or covered with low resolution Landsat imagery which is free).

Using this method you could easily produce a DVD that had Landsat imagery of an entire country and then high resolution imagery of areas of interest for a reasonable price. Worker in field would just need to plop the DVD in their laptop and they'd be good to go.

Jonathan Thompson said...

My response is up over at Aid Worker Daily. Here's the full link:

Rich Treves said...


aha. Seems a very sensible suggestion. I'm going to bear it in mind for future discussions, thanks for your comment.