Make-a-mashup!

Or, How to create a reference-able mosaic of satphotos

With Google Earth you can acquire screen-shots of Kabul, save them as files in a folder, and then splice them together as a “mashup”.

1. Start Google Earth.
2. Locate Kabul. You can do this by entering “Kabul” in the search field.
3. Turn off all the extra display features of Google Earth, such as terrain and boundary displays.

4. Zoom in to a fixed height; say, 1000m or 500m. This can be tricky work, zooming in and out by small increments until you get to exactly the right height. But it is worth the trouble, because later you can get more images at the same scale, so you can “mash” them together with older images you have grabbed.

5. Pan to the area where you will begin taking screen-shots. I recommend getting screen-shots in a series of columns, with the column-name representing the east or west position from a starting point. In Dasht-e Barchi, for example, I started with column w01, so the next ones to the west will be w02, w03, w04, etc. If I then wanted to grab screen-shots of areas to the east of this starting point, I would name eastward columns w00, e01, e02, e03, etc. Since I began taking screenshots at the northeast edge of the area at Kota-e Sangi, I worked south and then west in each series. The screenshots are therefore named:

screen-capture mosaic

As you can see, with the free version of Google Earth a hasty mosaic can be messy. I ‘panned’ by only using the arrow-keys rather than the mouse, so that I could shift exactly north or south. In theory you could put exact coordinates in the search bar; but that would cause Google Earth to jump back to 1000m and place a large, non-removable marker in the middle of the screen. So for the moment you will need to mosaic the images together by hand.

6. Get geographic information about your area. Google Earth gives extremely precise geographic information in the bottom status bar, as latitude and longitude down to the hundredth of a second. Note down the exact coordinates of obvious landmarks in your area. Later you will use this information to georeference your mashup when you import it into GIS. Examples:

Building in Kota-e Sangi, with cursor marking the NW corner

Here I have the cursor (in the form of a hand) centered over a landmark.

Here is a clipped extract of a Google Earth view of Kota-e Sangi. This shows the pavilion in the middle of the southeastern intersection, where taxis get gas. I have placed the cursor (in the shape of a hand) over the northwest corner of the pavilion. Google Earth shows me the exact geographic position of my cursor, in the bottom left corner of the display. In this case it is 34 degrees, 30 minutes, 44.50 seconds north by 69 degrees, 7 minutes, 5.50 seconds east. I note this down, and I have noted down four other landmarks as well:

NW corner of white gate-house on Pul-e Sokhta road, south of police compound:
34d 29′ 52.54″ N, 69d 06′ 53.87″ E

NW corner of northernmost Mille Bus parked along jog of Dasht-e Barchi road:
4d 29′ 54.61″ N, 69d 03′ 42.68″ E

NW corner of U-shaped building southwest of jog in Dasht-e Barchi road:
34d 29′ 37.14″ N, 69d 03′ 28.17″ E

7. Back in GIMP or Photoshop, I mark that point with a yellow pixel for two reasons. First, to visually record where I am locating landmarks for present and future work. Secondly, to give myself a target for georeferencing in QGIS. I zoom in and mark one pixel with yellow:

Kota-e Sangi building corner marked with a yellow pixel

Degree-conversion excercise

To georeference a mashup you need to convert the coordinates from degrees, minutes, seconds (dms) into decimal format. This is a good way to learn basic formula-functions in a spreadsheet program.

For simplicity I nicknamed each landmark, and then entered its dms coordinates in three columns. In the last column, named “decimal”, I entered a formula to calculate the decimal version of these coordinates. I divide the minutes by 60, and the seconds by 3600 to make them into decimal fractions of an hour, and then I add them together. The formula in Excel reads as: =B4+(C4/60)+(D4/3600)

label deg min sec decimal
Kota N 34 30 44.49 34.51235833
Kota E 69 07 5.50 69.11819444
Gate N 34 29 52.54 34.49792778
Gate E 69 06 53.87 69.11496389
Bus N 34 29 54.61 34.49850278
Bus E 69 03 42.68 69.06185556
U-house N 34 29 37.14 34.49365
U-house E 69 03 28.17 69.057825

Build the same table in a spreadsheet program and you will have a convenient tool for converting dms to decimal for all the landmarks you identify.