How to digitize map data using Google Earth

Google Earth is a free program. It may be downloaded here. Because Google Earth is one of several options for digitizing map data, it may be helpful to evaluate its advantages and disadvantages. A step-by-step manual to digitizing map data using Google Earth is available below.

Advantages

  • Imagery does not need to be georeferenced. The program communicates with the "Earth Server" to download and display imagery as you navigate from one place to another, or change heading, tilt, or other parameters.
  • Digitizing map data is an intuitive process. Data types do not need to be created in advance - you can create a point with one click of a button.

Disadvantages

  • Organization of data is clunky. You may drag or cut-paste only one record at a time. Additionally, data are organized in a series of folders in the program's "My Places" folder, which can be hard to find on your computer.
  • In order to download and display imagery, you must be connected to the internet. You may alternatively pan-cache but this requires advance planning, and even so, sometimes Google Earth may not open correctly if the program detects that the computer is not connected to the internet.
  • You may not use Google Earth to digitize data that were mapped in a region with low resolution. If the image that appears on the map is not a Google Earth image, it will not appear when you navigate to its location in Google Earth. In these cases, it will be necessary to use another program.

 

1. Open Google Earth.

2. Place the physical map on the ground. Remember that it is always more efficient to work with a partner. If you have not already created your folders, it is important that you do so before digitizing any of the map data. There is an added step to creating folders when workign with Google Earth. You must also create several folders in "Lugares Temporales" that correspond to the data types you wish to digitize. For example, consider a case in which we need to digitize a map in which appear latrines, community buildings, agroforestry plantations, and springs. In that case, you will need to create a folder for each of those data types. Even better, create a folder with the name of the community, and sub-folders for each of the individual data types, like this:

Here is a small part of a map that we will digitize in this exercise:

3. Navigate to the focus area in Google Earth. Sometimes it is helpful to use the "Buscar" box to seach for and fly to a nearby city. Remember, and town or landmark that appears on one the Google Map base layers is searchable. Once you have the same view in Google Earth as you (or your partner) has of the physical map you are ready to begin. A tip: it is nearly impossible to identify points from the physical map on the computer screen if the Google Earth image is "upside down". To rotate the screen perspective, use this feature:

4. Now we are ready to begin digitizing. Digitize by data type, beginning with the data type with the most features. In this case, it is latrines. Locate the first numerically referenced feature. In other words, locate the latrine labelled "#1". Here it is:

Here we notice two things:

  1. Latrines 1, 2, 3, 4, and 5 are all clustered together. It is much easier to digitize a map when adjacent points are numbered sequentially. The only way to do this is to number all the points of a given data type after they have been mapped. Resist the temptation to number points when they are first added to the map - they will jump from side to side and locating them in order during digitization will add much time and frustration to the process.
  2. The map (in this case, the photo that was taken of the map) is upside down compared to the image in Google Earth.

Up = North

Up = South

Use the tip from step 3 to rotate the screen perspective.

5. Locate point #1 in the Google Earth image. It may not actually appear in the image (for example, the house near the latrine may have been built after the photo was taken) and if that is the case, you will need to approximate. Once you have located the point, double-click on that spot on the screen. Google Earth will zoom in and center on that point. Now, click pushpin icon to add a new Placemark:

Alternatively, you may drag the pushpin to the correct location.

6. Name the point. In this case, we will name the point "1" because it is the first latrine. We do not need to name it "Latrine #1" because the point will be saved in the folder called "Latrines". Click "Aceptar" and verify that the point has been created within the "Latrines" folder. If not, drag it to that folder.

7. Repeat steps 4 through 6 for the rest of the latrines. They should all appear in the "Latrines" folder. Now, continue digitizing the rest of the data types, including lines and polygons. When you have finished, right-click on each folder and and select "Guardar lugar como...". Save it to the corresponding folder that you created as part of the first step of digitization.

You may also save each folder to your "Mis Lugares" but this is not necessary.