Things I Learned: The CZML Interval and Map Animation Time Control

By: noreply@blogger.com (antigrav_kids KD0FNR)

9 May 2024 at 13:23

I completed the first prototype of my CZML QSO mapping template for Datasette yesterday.

So far, I've implemented animation by using the interval property in CZML. Intervals can be attached to any (?) other tag it seems. I had to intuit what to do on a large degree based on the documentation.

{
    "id": "myObject",
    "someProperty": [
        {
            "interval": "2012-04-30T12:00:00Z/13:00:00Z",
            "number": 5
        },
        {
            "interval": "2012-04-30T13:00:00Z/14:00:00Z",
            "number": 6
        },
    ]
}

I used CZML polylines to display each QSO. Each polyline has a show property. I attached an interval to each QSO of one minute, like so:

    "id":"ea1ec",
    "name":"ea1ec",
    "polyline":{
      "positions":{
        "cartographicDegrees":[-106.5569121602622,32.371370471129424,0, -3.689354499999999,42.3504659,0]
      },
      "material":{
        "solidColor":{
          "color":{
            "rgba":[255, 0, 0, 255]
          }
        }
      },
      "show":[
          {
              "interval":"2024-03-14T23:29:00Z/2024-03-14T23:30:00Z",
              "boolean":true
          }
      ],
      
      "width":5,
      "clampToGround":true
    }
  }

The inference that the viewer (happily) made was that if the show property was true in the interval, then it must be false the rest of the time. (The default value of 'show' is true, so I was quite pleased with this implementation instead of the alternative where I might have had to define intervals for when I wanted the line turned off. This solution is far more intuitive to me.

I like several things about how this worked out with Cesium as opposed to other geospatial solutions available on the web. First, Cesium has animation available directly on the web as opposed to a desktop application.

Second, the player controls are available to the user and the player time range is specifiable in the CZML file, like this:

  {
    "id":"document",
    "name":"my first map",
    "version":"1.0",
    "clock":{
      "interval":"2024-03-14T23:28:00Z/2024-03-15T00:00:00Z",
      "currentTime":"2024-03-14T23:24:00Z",
      "multiplier":60,
      "range":"LOOP_STOP",
      "step":"SYSTEM_CLOCK_MULTIPLIER"
    }
  },
  {
    "id":"k6mdg",
    "name":"k6mdg",
    "polyline":{

The "clock" "interval" specifies the entire time available to the player. I need to modify my template a bit to make the beginning of the interval, (which is the time of the QSO at the moment), match the currentTime, (which is a few minutes earlier to provide a runway into the animation.) I suspect this may be what's causing me to have to click the play button twice to start the animation.

The final nice feature I'll point out today is that the Cesium Sandcastle makes it easy to play and experiment with both CZML and using JavaScript to control the viewer. You can even import CZML into Sandcastle sessions using the viewer's load method and statically hosted files. I used github as an easy platform to serve files from. (Keep in mind that new commits to files take about a minute to reach the raw page.)

On the downside, the documentation for CZML took me a while to get used to and required several mental leaps and experiments, some of which were incorrect.

Oh! And it turns out you can share and embed slide shows. I'm not an expert at these yet, but they seem promising.