Today we are looking at general features of the data.
First of all, we need to know what the colours mean. The colours represent the various available zoom levels in Google Maps, starting with low resolution at 7 to high resolution at 22. This can be roughly interpreted as follows:
|Low resolution ocean floor|
|High resolution ocean floor|
|Very low resolution satellite imagery (Landsat background imagery)|
|Low resolution satellite imagery (CNES/Spot Image background imagery)|
|High resolution satellite imagery (DigitalGlobe or CNES Astrium)|
|Aerial Imagery (higher resolution than satellite imagery)|
|Exceptionally high resolution Aerial Imagery|
Google Maps via API
To better understand Google Maps Zoom levels or to explore the data for a given location, you can use this page, which shows a full screen Google Map using the Google Maps API. It behaves a little differently from the standard Google Maps website. The standard Google Maps website restricts how far you can zoom in based on the imagery available, however, it always shows imagery. The above map, obtained via the API, lets you zoom in beyond the maximum prefered amount, and when you do it displays map tiles labelled “Sorry, we have no imagery here”.
You will notice in our map that there are noticeable bands in the data towards the poles at 60° and 75° latitude, both north and south. We believe these are an artifact of the Google Maps projection, which spreads out the poles, thus magnifying any imagery towards the poles and in consequence requiring less zoom in order to see a given resolution of imagery.
In our data collection we only went 80° north and south. Google Maps itself only goes to 85° north and south as a consequence of its map projection.
Google Earth has noticeable bands in the actual imagery at 80° north across Greenland and about 82.6° south in Antarctica. These are actual changes in the imagery datasets. The absence of high resolution near the poles may be due to the orbits of the imaging satellites or ‘sun angle constraints’.
The spider web of tracks across the oceans noticeable in our data reflect the paths of ships equipped with sonar for mapping the ocean floor. The tracks of higher resolution imagery are clearly visible in Google Earth and have in the past been mistaken for Atlantis or an alien base.
In our next post in this series we will be looking in a bit more detail at the various types of imagery found over land.