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Google Earth:  Combining multiple year captures

When you open Google Earth from inside AirPhotoSE, you should see the time line which offers imagery from different epochs in the upper left corner:



If you don't see it, close Google Earth from the file menu and try again.  Search for an area of interest with one of the search options such as Find by Name or Find In Grid after clicking on



You can turn the time line on and off if it was visible when GE first opened from the Action menu choice 



Drag the timeline bar to look at the images of different dates for the area chosen.



If you are looking for crop sites, these may be visible in one year but not in another:

  
2006      2009

To make a single image with the data from images of different dates, save each one to new image files without changing the position or scale (don't zoom, drag or move it!) after changing the date with the time line.    

Now use the polygon croping tool from the main toolbar to outline an area along field or other boundaries with single left mouse clicks and moving the mouse for each line segment





Close the polygon with a single right mouse button click:

  

In Options/Appearance, select a Black Background:



Then click Crop from the main menu:



  



If the cropped image is too dark, click on the Enhance item of the main menu



Then click on the "SaveAs" button to save the cropped image, or if you close it, you will be asked if you want to save it.  



The Selection Mask:

When you make a polygon selection, the values of the nodes of the polygon are used to create a mask which will permit writing or reading only to the parts of the image inside the mask.  This selection is written to a file which can be identified as belonging to the cropped image.

Warning:

You must use the SaveAs dialogue option or the resultant cropped images will not be properly masked.  The mask prevents a background from showing around the selected area.  Do not use the default Save button which will not copy the mask number.

The SaveAs dialogue will show the name of the source image from the year chosen with the special code number required for masking inserted in front of the extension:



Do not change this name!  Use only "SaveAs".  The code number is required so that the selected area file withe name of the source plus code number plus the extension '.sel' can be found.

Required:

Close all visible image windows before you attempt to use the cropped selections.

Use only the multiple image selection button to reopen the images. A background file and one or more cropped files must be selected.



to re-open the largest image with the most features to be used as a background and cropped features having the same name plus different selection code numbers which are to be inserted into the background:

..

Then, click on the Layers option on the main menu:



All the cropped images will be merged with the background.  Click on the File menu in the Layers window to save this result:



with Merge and Save Layers to preserve the calibration (geo-referencing) of the Google multi-date image.



Each selected cropped image from one date will be inserted in it's proper place in the background image:



Technical Details:

An uncropped background image has 2 files, the image and a calibration file. 

Each cropped image has 3 files.  The cropped image has the image file itself. It's calibration file is a modified version of that of the full image after cropping, and the third is a new "selection file"  which controls the parameters needed for masking out the unwanted background after cropping and insertion into the background image.  

All three must have the same mask identification number. This is inserted before the extension to synchronise blending.  When you crop, the name of the selection file with a unique mask number is computed first.  When the cropped image is saved using SaveAs, the image file name is modified so that the selection mask number is included to the left of the extension, and then it is written to disk along with it's calibration file that also has the mask number inserted

When the cropped files and the background file to which they are to be written are opened in the Layers option, names are searched using the selection mask names and placed in the background image at a position depending on the latitude/longitude stored in the calibration file.  If no selection file is found, then the cropped image will show a black background in layer mode, since a mask can not be computed.