logo

Google Earth

Like Google Maps, Google Earth probably doesn't need an introduction here. It's an excellent, free and feature-rich program (and online service) and the easiest way to display spatial data in KML format.

Google Earth can also be used to locate places in Tasmania. Zoom in to the spot on the satellite image and read the location under your mouse cursor (shown as a hand) in the status bar at the bottom of the Google Earth window:


Google Earth
 

The lat/lon shown, 42°10'35.06"S 146°45'14.37"E, is based on the WGS84 datum, which is the datum used natively by GPS satellites and Google Earth.

The lat/lon is given by default in DMS format, but you can change the display to decimal degrees or decimal minutes by going to 'Options' in the Google Earth 'Tools' menu and changing the format in the 'Show Lat/Long' box. Here you can also convert from lat/lon to the UTM system, but note that the UTM grid reference will be based on the WGS84 datum (see the conversions page).

The most important thing to note about a Google Earth lat/lon is that it's impossibly accurate. A lat/lon location to the nearest 0.01 second is an implied uncertainty of about ±15 cm (see the error page). You can't get that from a satellite image, which blurs as you zoom in close, and the status bar generally 'freezes' on a lat/lon at about 250 m eye elevation. At that point your placement of the cursor on the image will vary by metres, not centimetres.

There's also a location uncertainty which depends on how accurately Google Earth has placed its satellite image on its mathematical model of the Earth, a procedure known as georegistration. The accuracy of georegistration can vary from satellite image to satellite image and from date to date in an image series. Checking that georegistration can't be done at a desk — you need to do it in the field.

One method is to compare the the known location of an official government survey mark with its Google Earth lat/lon. I did this in 2012 with a survey mark in Penguin, and found that the Google Earth position was 2-3 m from the survey mark's actual location on that particular satellite image. Not bad, but definitely not ±15 cm.

You can also try getting a GPS reading at a point near the location of interest (an isolated rock, for example, or a track junction) which is clearly visible on the satellite image. If the difference between the GPS reading and the Google Earth lat/lon is more than the error in your GPS reading (see the error page), there could be a georegistration problem. In other words, if the difference is 20 m and you trust your GPS to ±10 m, don't use Google Earth to get a lat/lon for the location of interest in that area.

In summary, take a Google Earth lat/lon like 42°10'35.06"S 146°45'14.37"E with a big grain of salt, and round it off to 42°10'35"S 146°45'14"E.