Gallows Hill 1882

1882 map superimposed on Google Earth image

On 2 March 2012 Rob Drury demonstrated the use of Google Earth to display old maps over the image of the current city-scape. The steps were:

1. Download Google Earth – it’s free and can be found here: or go to ‘Google’, click on ‘More’ in the toolbar, and then ‘Even more’ in the drop-down box: Google Earth is in the list that appears and can be downloaded and installed.

2. Get to know the controls. Particularly useful for this purpose are:

  • The mouse – left and right buttons, and the combination of the left button and the shift key on the keyboard.
  • The fifth icon on the toolbar: if you hover over it with the cursor it says ‘Add image overlay’. If you left-click on this you get two things
    i. A box called ‘Google Earth – New image overlay’ and
    ii. a number of green markings on the map.

3. Acquire the maps you’re interested in. You can:

  • Scan them from an actual map,
  • Download them, from, for instance, ‘Old Maps’,
  • etc.
  • With most image software you can usefully trim the edges of the map.
  • If you have more sophisticated image software you can do clever things like making the map transparent and changing the colour of the lines to red.
  • If you are going to use the second method of fitting the map to Google Earth, this would be an appropriate point to add the markers (see 5b, below).

4. Use ‘Add image overlay’ to incorporate the map into your version of Google Earth:

  • First, turn off ‘Show Terrain’: Tools – Options – box about ¾ of the way down the left hand side.
  • Click on ‘Add image overlay’. The ‘properties’ dialogue box will appear together with the empty green box.
  • Scale the Google Earth map so that the green marking covers approximately the area of the map that you are going to add.
  • Click the ‘Browse’ button to look for your map, and click ‘Open’.
  • Your map will appear on the green markings.
  • It is useful to adjust the transparency so that you can see through the map to the original Google Earth image.

5. Fit the map on to Google Earth. Two alternative methods are suggested:

a. Adjust the map until it fits exactly. This is a ticklish job but gets easier with practice. It involves doing the following to your added map, using the mouse and the green markers:

  • Expanding or contracting: place the curser on the green markers at the corners of the image, press the left button, and crucially, keeping the shift key down (this keeps the proportions of the map correct: otherwise it will get distorted), move them in or out.
  • Adjusting the orientation: place the cursor over the diamond-shaped green marker that is (initially) towards the left and move as appropriate.
  • Adjusting the position of the map overall, using the curser over green cross in the centre of the map.
  • Final tweaking can be done without using the shift key.

b. Use place markers:

  • Print the map on A4 and identify 3 exact points that are visible on both Google earth and the print (corners of buildings etc). These should be spaced (ideally) in a wide triangle.
  • Using your image software spot the points you have selected on the scanned map (a red dot with the paintbrush tool, about 6 pixels dia) and save the additions.
  • In Google Earth use the place marker tool (2nd from left top row) to identify the same 3 points with a circle Icon (change the default ‘pin’ icon to ‘doughnut shaped icon using the dialogue box and alter the colour and size to suit, i.e., experiment. Then drag and drop icon over spotted location) remember to name the points (a,b,c) and click ok to see them in the Left hand ‘my places’ box.
  • Make sure the 3 points are ticked in ‘my places’.
  • Continue as in ‘a’ (above) but now you have clear reference points to focus on and align.

screen shot
Screen shot showing the dialogue box, green markers and panels

6. When you’ve finished, in the properties dialogue box you are using

  • Increase the transparency to ‘Opaque’.
  • Type in the name you want for the overlay (top of dialogue box).
  • Press OK.

The name of the new map should appear in the ‘Places’ panel to the left of the Google Earth image, under the heading ‘My Places’: if it’s in the wrong part of the list it’s easy to ‘drag and drop’ it into the right place.

‘Show Terrain’ can now be turned back on (Tools – Options: you may wish to increase the Elevation exaggeration to maximum of 3.)

7. To display the map in future, click on the square to the left of the map name. To adjust its opacity, click on the second icon on the bar at the bottom of the ‘Places’ window: a slider should appear. If more than one map is being displayed at the same time, highlight the name of the map that you wish the slider to apply to.

8. If you want to return to adjust the overlay, right click on its title and select ‘Properties’. The dialogue box and adjustable green outline will appear. Careful with those ‘handles’ though, you don’t want to undo all that good work! – Hence, a good idea is to employ point 10 below.

9. Next time you open Google earth the file will still be there in ‘My places’ until you delete it.

10. The overlay(s) you have created can be stored (right click on overlay title and ‘save as’ – choose a place to save). They can also be sent as an attachment in an e-mail. The attachment opens Google Earth on the recipient’s computer and the overlay appears ready ticked in his/her ‘My places’

by Rob Drury and Tom Zadik ©

2 March 2012