3/20/2023 0 Comments Goldencheetah weather dataThis is still about 33% higher than reality, but it's a lot closer than the GPS-only result.īut surely we can do better: how can we make the calculations even less sensitive to the jiggles and wobbles that are inherent in GPS tracks? This time, the total gain/loss comes out to 1434 meters. The green DEM line is much more reasonable, and we can calculate its gain/loss using the same spreadsheet as in the previous example. Look at the ridiculous peak in the GPS line at around 8.2 km, where the GPS thought 60 meters were gained and then 40 meters lost, when in reality the trail steadily gained about 20 meters: If we simply replace the GPS elevation information with data from a Digital Elevation Model (in this case, the USGS's 30m-resolution National Elevation Dataset for the U.S.), the data looks much "smoother," because the elevation number isn't jumping up and down at random like it does on a GPS. There's just too much noise in the GPS data. Recall that the correct figure is about 1080 meters if we use the raw data directly from the GPS, it tells us that we climbed twice as much as we really did! This is a difficult hike, but it certainly doesn't climb 2000+ meters. Simple, right? But here's the problem: if we continue this calculation for all 3805 trackpoints, we find an elevation gain of 2326 meters (and a loss of 2322m, because the GPS device can't even agree with itself about the elevation of the starting/ending point). You're carrying a GPS device that records the elevation every 5 meters, but the signal wanders up and down a bit it thinks that each point is 2m above or 2m below the previous one. Here's a "reductio ad absurdum" example that illustrates some of the problem: Imagine that you're walking along perfectly level ground for 100 meters. If you know exactly where your track changed from going uphill to going downhill (or vice versa), this is the most accurate method for calculating elevation gain you don't have to know the elevations of all the points, just the places that it changed direction. Manual calculation: If all else fails, you can find elevations to some of your trackpoints manually - using known points from high-quality surveys or topographic maps - and do some simple math. Generally, altimeters do a pretty good job with relative elevation changes, so you probably don't want to replace barometric data with DEM data if your goal is calculating gain/loss. This can be quite accurate, if the barometer is properly calibrated and if the weather doesn't change. Barometric data: Many GPS devices come with a built-in barometer/altimeter, and they use the barometer for elevation data - sometimes using GPS to perform auto-calibration along the way.High-resolution DEMs can produce pretty good estimates, especially for roads and other places where the trees have been cleared and the remote-sensing spacecraft can see clearly all the way to the surface. DEM (Digital Elevation Model): A DEM is a database of known elevations for many points on the earth - ideally millions or billions of points - derived from topographic surveys and/or from orbiting radar or lidar.In a canyon or among trees, fewer satellites will be available (and/or the signal may bounce around), and it will be even worse. Because the GPS satellites are scattered throughout the sky in a more-or-less horizontal plane, the vertical accuracy is simply not very good. GPS data: This is the most common but least accurate.There are four ways to get elevation information into a GPS data file: The difficulty lies not in the calculations themselves, but in the fact that the elevation data is never 100% accurate. Let's be clear: calculating elevation gain is difficult. « GPS Visualizer Tutorials Tutorial: Calculating Elevation Gain
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