The simplest way is to click on the button labeled Validate random points. This will automatically zoom the user into a degree confluence point and show the outlines of the pixels of the three land cover datasets (i.e. MODIS, GlobCover and GLC-2000), indicating on the right what the land cover types are in the pixels that contain the confluence point as shown in Figure 1. Based on what the user sees on Google Earth, they should indicate whether the legend description for each land cover product is a good or bad match or whether they are not sure due to insufficient visual information. A sliding bar to indicate confidence in the assessment of land cover in the land cover products is also provided. The Submit button will write this information to a database. The user can then continue to further assess the land cover at random points.
Figure 1: Assessing land cover at a random location on the Earth’s surface for MODIS, GlobCover and GLC-2000 using Google Earth with proposed corrections to the land cover information and a confidence scale bar
Users can also undertake the land cover assessment process by automatically having Geo-Wiki zoom into the location under which they are registered, e.g. workplace, home, etc. Land cover can then be assessed in an area with which the user is familiar, where points will be selected randomly across a 100 km grid around the user’s location. Alternatively, the user can zoom into any location on the Earth’s surface and undertake land cover assessment. The buttons in the toolbar shown in Figure 7 (located in the upper left hand corner of the screen) allow the user to choose a location on the Earth’s surface but requires the user to be zoomed in sufficiently before the outlines of the land cover pixels will appear. The first button in the toolbar with an arrow indicates normal mode. Double clicking anywhere on the Earth will zoom the user in further. The next three buttons in the toolbar are used to choose a point. The second button from the left is the information button. Clicking anywhere while this button is enabled will highlight in red the points at which the land cover will be assessed. Double clicking on one of these red points will zoom the user into that point and show the pixels for land cover assessment. The third button, which contains a crosshair, does the same thing but does not require clicking on a red point. The fourth button allows the user to drag the cursor across an area and highlight a point within that area for land cover assessment. The disagreement maps, like that shown in Figure 5, can also be used to help focus on areas where more validation is needed. Finally, users can define a systematic grid of points for assessing land cover and adjust the granularity of the grid as shown in Figure 6. This feature is accessed by clicking on the View Profile button followed by the Custom Area link. This feature will be extended in the future to add the possibility of random data collection within the chosen area or those locations with the highest disagreement.
Figure 2: Example of a 0.5 degree systematic validation grid over western Africa in areas of high uncertainty in cropland extent.
Last edited: 08 August 2012
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