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CZML Structure
NOTE: This is a work in progress and reflects our plans NOT our current capabilities.
CZML is a subset of JSON, meaning that a valid CZML document is also a valid JSON document. Specifically, a CZML document contains a single JSON array where each object-literal element in the array is a CZML packet. A CZML packet describes the graphical properties for a single object in the scene, such as a single aircraft.
Note: we use javascript comments in these examples even though comments are not technically allowed in JSON.
[
// packet one
{
"id": "GroundControlStation"
"position": { "cartographicDegrees": [-75.5, 40.0, 0.0] },
"point": {
"color": { "rgba": [0, 0, 255, 255] },
}
},
// packet two
{
"id": "PredatorUAV",
// ...
}
]Each packet has an id property identifying the object it is describing. IDs do not need to be GUIDs - URIs make good IDs - but they do need to uniquely identify a single object within a CZML source and any other CZML sources loaded into the same scope. We'll talk more about scopes later in this document.
If an id is not specified, the client will automatically generate a unique one. However, this prevents later packets from referring to this object in order to, for example, add more data to it.
In addition to the id, packets contain zero or more (but usually one or more) properties defining the graphical characteristics of the object. In the example above, we've specified that the "GroundControlStation" object has a fixed position at WGS 84 longitude -75.5 degrees, latitude 40.0 degrees, and height 0.0 meters, and that a blue point (dot) is drawn at its location.
There are many standard properties defined for CZML, including properties for adding points, billboards, models, lines, and other graphics to the scene. The available properties are described in detail on the CZML Content page. On this page, we are primarily concerned with how the data is structured. For example, we describe how a property can be specified such that it has two different values over two different intervals of time.
In the most general case, the value of a CZML property is a JSON array, where each element in the array is an object literal defining the value of the property for a different interval of time. The interval described by any given object literal in the array is specified using an ISO8601 interval string in the interval property.
{
"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
},
]
}Here we define the someProperty property over two intervals, the first from noon to 1:00 PM UTC where the value of the property is 5, and the other from 1:00 PM to 2:00 PM UTC where the value of the property is 6. The value will change instantly when crossing the boundary between two intervals. We use number to indicate the value because this is a number-type property. Some properties, notably properties that represent position, allow the value to be specified in multiple formats, such as a Cartesian X,Y,Z position or a Cartographic longitude, latitude, height position. The CZML Content page lists the types of data supported for each property, and the value names to use with each.
The interval property is optional. If it is not specified, the interval is assumed to span all time. It doesn't make much sense to specify multiple infinite intervals, or intervals that overlap in general, but if you do, the one later in the CZML file or stream takes precedence.
In the common case that the property has a value over just one interval, the interval list array can be omitted entirely.
{
"id": "myObject",
"someProperty": {
"interval": "2012-04-30T12:00:00Z/14:00:00Z",
"number": 5
}
}Just as before, the interval property can be omitted if it spans all time. For properties with simple values, like the number property shown above, and with a single value for all time, the value can be given even more compactly:
{
"id": "myObject",
"someProperty": 5
}This abbreviated notation is valid for any property whose value can be represented with one of the simple JSON data types: string, number, or boolean.
More complicated composite values, such as a Cartesian position or a color, are represented using JSON arrays. For a Cartesian position, the array has three elements, corresponding to the X, Y, and Z components of the position, respectively.
{
"id": "myObject",
"someComplexProperty": {
"cartesian": [1.0, 2.0, 3.0]
}
}Composite values must always be specified within an interval, even if that interval is infinite as shown here. If the value, [1.0, 2.0, 3.0], were allowed as the value of the complexProperty directly, it would be necessary for a client interpreting the CZML to look at the contents of the array to determine whether the array is a list of intervals or a single value. So, for simplicity, we do not allow this.
The CZML Content page describes how various types of data are encoded in arrays.
So far, we've discussed how to specify a single value for a property for all time, and how to specify different values for a property over different discrete intervals. Some properties (and the CZML Content page will tell you which) also allow you to specify time-tagged samples which the client will interpolate over to compute the value of the property at any given time. Times are specified using ISO8601 strings.
{
// ...
"someInterpolatableProperty": {
"cartesian": [
"2012-04-30T12:00Z", 1.0, 2.0, 3.0,
"2012-04-30T12:01Z", 4.0, 5.0, 6.0,
"2012-04-30T12:02Z", 7.0, 8.0, 9.0
]
}
} Here we're specifying that the value is [1.0, 2.0, 3.0] at noon, [4.0, 5.0, 6.0] one minute later, and [7.0, 8.0, 9.0] a minute after that. If the client's current clock is 30 seconds past noon, the value will be a linear interpolation between [1.0, 2.0, 3.0] and [4.0, 5.0, 6.0], or [2.5, 3.5, 4.5].
For succinctness, times can also be specified in seconds since an epoch. While this is potentially less precise than specifying each time using an ISO8601 string, it is usually more than sufficient when the samples span less than a day or when the offsets are whole numbers of seconds.
{
// ...
"someInterpolatableProperty": {
"epoch": "2012-04-30T12:00Z",
"cartesian": [
0.0, 1.0, 2.0, 3.0,
60.0, 4.0, 5.0, 6.0,
120.0, 7.0, 8.0, 9.0
]
}
}Finally, properties specified using time-tagged samples have some additional, optional sub-properties controlling interpolation.
{
// ...
"someInterpolatableProperty": {
"epoch": "2012-04-30T12:00Z",
"cartesian": [
0.0, 1.0, 2.0, 3.0,
60.0, 4.0, 5.0, 6.0,
120.0, 7.0, 8.0, 9.0
],
"interpolationAlgorithm": "LAGRANGE",
"interpolationDegree": 5
},
} The interpolationAlgorithm specifies the algorithm to use to interpolate a value at a different time from the provided data. The available algorithms are described on the CZML Content page. The interpolationDegree property specifies the degree of the polynomial to use for interpolation. For example, 1 means linear interpolation and 2 means quadratic interpolation. If these properties are not specified, linear interpolation is used.
Putting an entire CZML document in one big JSON array makes it difficult to load the document incrementally. Today's web browsers allow some access to a stream before it is complete, but parsing and interpreting the incomplete data requires slow and cumbersome string manipulations. To faciliate high-performance streaming, CZML may also be streamed using modern browsers' server-sent events (EventSource) API. When using this API, each CZML packet is streamed to the client as a separate event:
event: czml
data: {
// packet one
}
event: czml
data: {
// packet two
} As a result, the browser raises an event when each packet is received, containing the data for just that one packet. This allows us to incrementally process CZML data with excellent performance.