Using Google Earth as a Map Viewer

For many, Google Earth (GE) is a virtual globe with world coverage of satellite and air photo imagery. Users can download and use the free program to view their home and neighbourhood, take a peek at volcanoes and islands unheard of before. With image resolution varying from 15cm to 15m, some lucky users can zoom right into their backyard and use the ruler tool to measure the size of their pool and their neighbour's pool as well. In fact, many hours can be spent exploring what Google has to offer - 3-D buildings, Digital Elevation Models (DEM) of mountains, and even non-imagery features such as weather, places of interest, traffic information, and much more.

What many internet users don't know however is that GE in fact offers much more than what can be seen on the surface. By using some of GE's tools, one can create maps and views with limitless features. One can customize the virtual globe to displayanything. Users can add their own imagery, their own GIS files and superimpose them over GE's own features.

Adding Your Own Images

Users can add their own images into GE and correctly align the image to the earth's surface by supplying the image's spatial coordinates. If the image is already georeferenced, the user can obtain the coordinates of the image's four corners from a GIS program and enter it into the image properties in GE.

Orthoimage of a portion of the City of Kitchener, 2000.
Image courtesy of the City of Kitchener
Non-georeferenced images (like personal photographs or scanned maps) can be georeferenced by overlaying the photo over Google's imagery and adjusting the image accordingly (by extending or retracting it).

The image is added into GE as an Image Overlay (Add -Image Overlay). Users may find this tool particularly useful when comparing aerial photography taken throughout the years. Once aligned correctly, users can turn the images on and off to visualize changes in the landscape. The transparency feature can also be used in order to see two images superimposed over one another.

Adding Your Own GIS Files

The world is your oyster, as it were, when it comes to adding external files into GE. Any geospatial file can be added into the program, provided that it is in a format that is readable by GE. Currently, GE only accepts geospatial files in KML (Keyhole Markup Language). Fortunately, there are several free Shapefile-to-KML converters available for download. If your file is not a Shapefile originally, then you can use another program that will convert it to Shapefile. An excellent shapefile-to-KML converter is the Shp2kml which is a stand alone product that allows for the customization of the layer – the user can specify symbols (colours, width), classification field, and labelling preferences. Shp2kml supports Shapefiles in Lat/Long or UTM, in points, lines, and polygons.

In Google Earth, the user adds the KML file by clicking on Add from the File menu.

Buildings, 2006. Data courtesy of City of Kitchener.

Shp2kml was used to convert the original building Shapefile to a KML. Symbols and categories were defined and retained during conversion. User selected individual fields to appear in the attribute 'balloon'.

Connecting to a WMS

Users who do not have interest in, or access to geospatial files, may opt to use GE's Web Map Service (WMS) client feature to connect to any WMS URL. A WMS allows users to access maps or individual files from any web server that offers it. Users can combine the maps from one or more servers as well as use it with GE's imagery. Types of information available from WMS can include geology, population, topography, weather, imagery and more.

A connection to a WMS is established in GE from the Add menu. Clicking on Image Overlay – Refresh – and on WMS Parameters. The user can select from the list that is readily available, or the user can Add a WMS URL to connect to a specific service s/he is interested in.

Streets are added into GE. Depending on the
transparency level, the user may opt not to have
any of Google's imagery exposed.

Topographic layers added into GE: Parks, Water, Buildings
As can be seen in the images above, Google Earth can be used as a map viewer using and manipulating external geospatial files. Users can combine their own data with Google's, or opt to display their data alone. The kinds of information that one can display in GE is limitless, and hence serving the needs of both GIS-savvy and non-GIS users.

By Eva Dodsworth
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