( UPDATE 2/2/2015 – In a recent post I mentioned that Google Earth Pro is now available as a free version. Most building heights can be measured much more accurately using the built-in tools that come with it. )
There are many times when you’re studying an arial photograph of an area and realize it would be great to have some idea of how high a building ( or tree or other object ) is.
I figured out a way to do this using Google Sketchup that’s fast and pretty accurate, depending on the quality of the photograph of the object.
First, you want to find the location in Google Earth, not Google Maps. The reason is that in Google Earth, each photograph will have a date stamp in the bottom left corner. You need the exact date the photo was taken.
Then, with a new window in Sketchup open, click the “Add Location” button in the toolbar at the top of the window. It’s a little icon that looks like a folded map. Or, you can go under ‘File’ to ‘Geo-Location’ to the ‘Add More Imagery’ sub-menu.
Navigate to the same location on the map and you’ll see that it’s the same arial photo as in Google Earth. Zoom in as close as possible and adjust the grab frame to include only what you need. In the example, we’ll grab a photo of the New York Street backlot facades at 20th Century Fox Studios.
Click the ‘Grab’ button in the upper right to bring it into Sketchup. Open the ‘Layers’ panel in the ‘Windows’ sub-menu. You’ll notice that there are two new layers, Google Earth Snapshot and Google Earth Terrain. For the most accuracy, be sure the Google Earth Terrain layer is turned on by checking the box.
Now, under the Window menu, you want to check Shadows to open the Shadows Setting dialogue box. Enter the day and month of the Google Earth date stamp and click the box in the top left to turn on the shadows. Be sure the ‘on ground’ box at the bottom is also checked.
Now, using the photo as a guide, draw the outline of one of the buildings and use the push/pull tool to pull it up slightly. You will now see the shadow of the form in relation to the shadow cast in the photo.
Using the Time slider in the Shadow Settings box, slide it back and forth until the two shadow lines meet.
Then use the push/pull tool once again to pull the building up until the top of its shadow matches the photograph shadow. Pull the other surrounding buildings up to meet their shadows as well.
The accuracy of the final measured height will depend on a number of factors: the quality of the photo, how close the model terrain is to that at the actual site, how accurately you have drawn the position of the footprint of the object, and the shadow length ratio. You’ll get better results if the shadow length is longer than the actual object height. Of course if the shadow is obscured or non-existant, you’ll have to find the height by going there.
In my next post I’ll show you three different ways to find a building’s height; high-tech, low-tech and no-tech.