elevation

Drawing contours with altitude data

August 12, 2016

Thank you to GEB reader ‘DJ’ for suggesting this idea in the comments of our post on simulating lakes. It turns out that the algorithm for drawing contours given a regular array of altitude data is actually very simple. We used the algorithm known as ‘Marching Squares’ as described on Wikipedia. For this first attempt we are not going to bother with some of the extras, such as smoothing the final result, or dealing with saddle points.

The main difficulty is getting hold of altitude data. Google offers the Google Maps Elevation API, but it has significant restrictions, including usage limits and most important, rules about what you may do with the data, especially this line:

The Google Maps Elevation API may only be used in conjunction with displaying results on a Google map. It is prohibited to use Google Maps Elevation API data without displaying a Google map.”

If we understand that correctly as well as other restrictions regarding not storing the results, then we are not allowed to use the API to create KML files for use in Google Earth.

We had a look around for other elevation API’s but most of them have similar restrictions, i.e., you can only use them with the providers maps. There are a few that might be less restrictive, such as the ones provided by the USGS and Mapzen, but we will need to investigate them further to double check the permissions as well as how to access the data. There may be others out there. If any of our readers knows of a no-restrictions elevation API, please let us know in the comments.

For now, we have decided to run some tests with the Google Maps API and try to comply with the restrictions.

To use it, start by drawing a polygon in Google Earth, outlining the approximate area you are interested in. Save the polygon as a KML file and select it below. Choose an altitude (in metres) at which you want to draw a contour. Click the ‘Draw contour’ button.
It takes about a minute because of restrictions imposed by the Elevation API on the number of queries you are allowed to make in a given time. For this reason we have used a fairly coarse grid of 100 x 100 points.

KML polygon of the area of interest:

Altitude: m

The final result should be a black contour shown on the Google Map above. As an example, we tried the lake from our post on simulating lakes:

A contour outlining the proposed Batoka Gorge lake

Google Earth Elevation Profiles

April 21, 2015

An often overlooked feature of Google Earth, elevation profiles were first introduced in version 5.2. The feature is easy to use, all you need is a path selected in your ‘My Places’ then go to the ‘Edit’ menu and select ‘Show Elevation Profile’.

If you want the elevation profile of a slice through a mountain or valley, simply draw a straight line using the ‘Add Path’ tool on the Google Earth toolbar. But it is not restricted to straight lines and much more often you will be interested in the elevation profile of a hike you are planning, or bicycle route. In this case, you can draw out the path as before, or if it is a route on roads/paths already marked on Google Earth, you can use the Get Directions feature right-click (CTRL click on Mac) on the blue line and select ‘Show Elevation Profile’.

Elevation profile of a route in the Swiss Alps.

If you hold your mouse over the elevation profile, it will show a red arrow on the map marking the location, and also display the height above sea level and gradient at that point.

Keep in mind that Google Earth’s elevation data is not very high resolution and should not be taken as anything more than a rough guide. We also discovered that on the route shown above there were several tunnels and bridges and the route in Google Earth that is used for the elevation profile follows the ground and not the actual road surface resulting in large bumps and dips in the elevation profile.

USGS 3D Elevation Program

October 16, 2014

The US Geological Survey (USGS) has a program called the 3D Elevation Program (3DEP).

From the USGS website:

The 3D Elevation Program (3DEP) initiative is being developed to respond to growing needs for high-quality topographic data and for a wide range of other three-dimensional representations of the Nation’s natural and constructed features. The primary goal of 3DEP is to systematically collect enhanced elevation data in the form of high-quality light detection and ranging (lidar) data over the conterminous United States, Hawaii, and the U.S. territories, with data acquired over an 8-year period. Interferometric synthetic aperture radar (ifsar) data will be collected over Alaska, where cloud cover and remote locations preclude the use of lidar over much of the State.

We believe that once completed the data will be placed in the public domain, which will mean that Google will use it in Google Earth. It is not clear how the resolution will compare to the 3D imagery that Google has been rolling out for cities that is created using a different technique. It would certainly not replace Google’s 3D imagery, as that requires not just a 3D model, but also photos of the different faces of the models, which will not be provided by the USGS data. However, it would certainly significantly improve the 3D landscape for areas where Google has not created 3D mesh.

Google has worked closely with USGS in the past. The Earthquake layer in Google Earth is provided by the USGS, as was the imagery used in Google Earth Engine to create global yearly images and a timelapse of the whole earth.

High-resolution lidar image of Mount St. Helens, Washington. Courtesy of USGS. A downloadable high resolution poster of this image can be found here

This article in the Washington post shows a sample of the new terrain maps, which clearly shows trees and is reminiscent of Google’s 3D imagery. However, this article from The American Surveyor shows a sample in which the trees hardly seem to feature. I guess we will have to wait for the first actual releases of data to see what the results are.

Measuring distances with the Google Earth Ruler

August 6, 2013

The “ruler” tool in Google Earth has always been useful, and it has seen a number of improvements over the years. To get started with it, simply go to [Tools] –> [Ruler] in Google Earth and it’ll open up for you in a small window.

Here are a few things you can do with it:

Measure the length of a field
To measure the length of a field (or driveway, or any other straight line), simply use the “line” feature in the ruler. Choose your unit of measure (inches, meters, miles, etc) and then click the two points on the ground you’d like to measure. That’s it — it’s very easy!

Measure a running/biking route
I have a GPS-enabled watch that can track how far I run, which is quite cool. However, I usually want to know how far my route is before I head out, and the Ruler’s “path” tool is great for that. By choosing the path, and then clicking various points along my route, I can see how far it’ll be. As long as you plot your points carefully, it will be remarkably accurate. I also change the setting to “miles” for this one, since that’s how I typically measure my runs. This would also work well for biking, hiking, or just walking. You could potentially use it to measure driving distance, though the standard “directions” feature would work better in most of those cases.

Save and edit your path
Once you’ve created your path, you might want to save it for future reference. Simply click the “save” button in the Ruler’s window and it will save the path to your “Temporary Places” folder. You can edit the path in the future by right-clicking on the item from within your “places” panel and choosing “properties”.

Get an elevation profile
One of the great new features unveiled back in Google Earth 5.2 was “Elevation Profiles”. This allows you to quickly see the elevation changes over a particular area. If you save your path in Google Earth, you can view the elevation profile for it very easily. Right-click on the item you saved in your “places” panel and choose “Show Elevation Profile”.

In this example, we’ll hike up Kennesaw mountain, and then follow the service road to get back down. You can see that the elevation profile shows the steep ascent, followed by the more gradual descent on the way back down.

The Ruler tool in Google Earth can be quite useful, so head into [Tools] –> [Ruler] to try it for yourself!

New Terrain Mode for Google Maps

November 27, 2007

Google has announced a new mode called Terrain for Google Maps which lets you see relief maps of 3D terrain. This is a very useful mode if you are going hiking, mountain biking, skiing, etc. in rough terrain areas. Or, if you just want to get an idea of the lay of the land when you’re looking at the map. They also got rid of the “Hybrid” button which showed roads and labels when you were in Satellite mode. Now, when you click on Satellite, you get a pop-up choice to add “Labels” – which does the same thing as Hybrid did. Check out this new terrain layer for your favorite hilly areas. The detail is quite good in most of the places I’ve checked. Zoom in closer for more detail.
One of the best features of Google Earth is that it is a 3D virtual globe, not just a 2D map. So, when you tilt your view, the terrain is represented by actual 3D data. So, you can literally see details of the terrain like you were standing/floating there. However, the terrain is only as good as the resolution of the 3D data. Most of the world is represented in Google Earth by data from US Space Shuttle missions at 90 meters resolution. You may not see small hills, boulders, etc. In the US, New Zealand, Canaries, Switzerland, and parts of Canada – Google has added new higher resolution 3D terrain. Most of the new stuff is at 10 meter resolution. In West Virginia it is at 3 meters resolution! Google seems committed to continuing to add more areas of the world with better 3D terrain data. And, they are now adding terrain information to Google Maps as well. The more information the better!
By the way, you can also look at shaded relief maps in Google Earth as an overlay. See the example below. And, you can also overlay USGS topographical maps to Google Earth.
Related: