PHP Looping through a GPX to calculate total ascent/descent of a track

Question

I want loop though a gpx file and calculate the total ascent and descent. I have a function that can calc the difference in elevation between two sets of lat long points and I've set up simplexml to read & loop through the gpx file trkseg points.

The problem is, that this is not accurate (really not accurate as it is in real).

This two lines will result in different ascent and descent, as it is in the real life:

$total_ascent += ($val - $last_elevation);
$total_descent += ($val - $last_elevation);

Does somebody know, how to calculate more accurate the total ascent and descent of a track?

This is my current code snippet to calculate it ($track_elevations is an array with elevations of whole track):

if (!empty($track_elevations)) {
    $total_ascent = $total_descent = 0
    $lowest_elevation = $highest_elevation = $last_elevation = null;
    
    foreach ($track_elevations as &$val) {
        if (!is_null($last_elevation)) {
            if ($last_elevation < $val) {
                $total_ascent += ($val - $last_elevation);
            }
            elseif ($last_elevation > $val) {
                $total_descent += ($last_elevation - $val);
            }
        }

        if (is_null($lowest_elevation) or $lowest_elevation > $val) {
            $lowest_elevation = $val;
        }

        if (is_null($highest_elevation) or $highest_elevation < $val) {
            $highest_elevation = $val;
        }
        
        $last_elevation = $val;
    }
}

Example of $track_elevations array:

$track_elevations = array(
    327.46,
    328.27,
    329.32,
    330.11,
    329.46,
    329.39,
    329.68,
    331.04,
    333.23,
    333.46,
    332.97,
    332.88,
    332.99,
    332.75,
    332.74,
    334.01,
    333.62
)

In real, I was riding bike on the flat road. But my snippet of code will calculate, I have ascended and descended couple meters. Maybe I should add there some limitation of precision between two elevations in a row...

What I want to achieve:

I will try to explain it more better - when I ride f.e. 20 km on flat road (almost with no ascent and descent), the total ascent and descent should be close to 0. But when I sum $track_elevations (like in my snippet of code), I will get in $total_ascent and $total_descent f.e. 500 meters... Its because of between each array element in $track_elevations is difference couple centimeters, but I am still riding on the flat road... And in the total sum it will gather to a large number of meters... Hope now it is more clear.

Answer 1

The problem is that if you calculate $lastHeight - $track_elevations[$i] for ascentet you get values lower than 0 and thats why you get high differences between $total_asc and $total_desc.

<?php
$track_elevations = array(
    327.46,
    328.27,
    329.32,
    330.11,
    329.46,
    329.39,
    329.68,
    331.04,
    333.23,
    333.46,
    332.97,
    332.88,
    332.99,
    332.75,
    332.74,
    334.01,
    333.62
);

$lastHeight = $track_elevations[0];
$total_ascent = 0;
$total_descent = 0;
for ($i=1;$i<count($track_elevations);$i++) {
    if ($track_elevations[$i] > $lastHeight) {
        $total_ascent += -1 * ($lastHeight - $track_elevations[$i]);
    }
    else {
        $total_descent += ($lastHeight - $track_elevations[$i]);
    }
    $lastHeight = $track_elevations[$i];
}
echo $total_ascent ."\n";
echo $total_descent . "\n\n--\n";
echo $total_ascent  - $total_descent . "\n";

--

$ php -f gpx.php
8.1
1.94

--
6.16

-- manual calculates values:

Ascent:
0.81
1.05
0.79
0.29
1.36
2.19
0.23
0.11
1.27
---
8.1

Descent:
0.65
0.07
0.49
0.09
0.24
0.01
0.39
---
1.94    

Answer 2

Are you sure this is "flat" ? Can you provide full data set? Also I don't think that this is a precision issue from PHP. Your datapoints have only two digit precision. Maybe the sensor is rounding ?

Answer 3

Even if you ride a perfectly flat road, elevation data gathered from GPS will always go up and down a little. GPS is not very accurate with altitude, even in perfect conditions.

If you use raw GPS input to calculate the total amount of climbing/descending, these little errors will sum up quickly with every single point and give completely unrealistic results.

There are a number of options to get better results, but they are all more or less complex: You could "smooth" the elevation curve with proper algorithms. You could discard GPS altitude data and replace it with pre-recorded altitude data for all coordinates on earth (called SRTM data).
You could use the SRTM data and mix it intelligently with your own GPS data.

In any case, getting proper altitude values is a difficult topic. You could try the simple way of not counting differences below 10m or some such, but it will most likely not lead to good results.