Earth-mapping service constantly updates images, but it can confuse


POSTED: Sunday, July 19, 2009

Every so often, the Star-Bulletin receives an urgent call or message asking about the light aircraft that crashed in Honolulu Harbor and is floating just below the surface. Sometimes the site is referred to as Pearl Harbor, but it's really Honolulu Harbor, in the channel off Sand Island.

Look! You can see it right there in Google Maps or on Google Earth!

At first glance it sure looks like a Cessna in the water, drifting right where ships go in and out of the harbor.

A second glance, however, reveals what is actually going on. Google's earth-mapping service is constantly updating satellite images of the planet, using the best available to create their maps. To create a continuous map, the images are overlapped digitally, particularly in neutral areas like water.

At different points in time, satellites took images of Honolulu Harbor. In one image a light plane was caught on approach to runway 26-R at the airport. It was digitally composited with another image that had no airplane, and the pixels of both images were mixed—with the result of looking like an airplane underwater.

Just a couple of hundred yards away, the digital overlapping is more obvious, with a Coast Guard cutter half-missing at her berth, and Sand Island just partly in bloom.

No emergency here, folks. Move along.