Mapping XML unit codes and observed timestamps

A measurement needs a value, a unit, and a timestamp. This example maps the latest observation from one weather station to the WeatherObserved Smart Data Model. Each reading gets a unitCode and an observedAt, so later fetches can add observations for the same station instead of replacing earlier readings.

Get the data

The feed is the latest surface observation for Ljubljana Bežigrad, from the Slovenian Environment Agency (ARSO):

curl --fail --location --output data/observation.xml "https://meteo.arso.gov.si/uploads/probase/www/observ/surface/text/sl/observation_LJUBL-ANA_BEZIGRAD_latest.xml"

Get the schema

WeatherObserved is a published Smart Data Model. Download the catalog once:

cassiopeia sdm download

The record shape

The document contains one station inside an envelope. Feed-level fields come first, followed by one metData element containing the observation. Since nothing repeats, Cassiopeia keeps the document as one record and exposes the measurements under metData. A reading pairs a value with its unit and description:

<metData>
    <domain_id>65142878</domain_id>
    <domain_lat>46.0658</domain_lat>
    <domain_lon>14.5172</domain_lon>
    <domain_title>LJUBLJANA/BEZIGRAD</domain_title>
    <valid_UTC>21.08.2026 10:00 UTC</valid_UTC>
    <t_var_unit>°C</t_var_unit>
    <t>23</t>
    <rh>80</rh>
    <msl>1011</msl>
    <ff_val>0.2</ff_val>
    <dd_val>54</dd_val>
    <vis_value>20</vis_value>
</metData>

The mapping reads each field as metData.<name>.

unitCode is a code, not a symbol

The feed prints units as symbols such as °C, hPa, and m/s. NGSI-LD's unitCode is different: it uses the UN/CEFACT Common Code for the unit (ETSI GS CIM 009 v1.9.1 clause 4.5.2.2). Each field has a fixed unit, so the mapping uses a constant code:

temperature: {
    source: "{{ metData.t }}",
    type: "Property",
    transformation: "float",
    properties: {
        unitCode: {
            source: "CEL",
        },
        observedAt: {
            source: "{{ metData.valid_UTC | replace(from='.', to='/') | replace(from=' UTC', to='') }}",
            type: "Property",
            transformation: "datetime",
        },
    },
},

unitCode and observedAt are declared under properties, like other attribute-level properties. NGSI-LD reserves those names, so they are emitted as qualifiers on the value rather than as ordinary nested properties. The codes used here are:

Quantity	Feed symbol	unitCode
temperature, dew point	°C	CEL
relative humidity	% (as ratio)	C62
atmospheric pressure	hPa	A97
wind speed	m/s	MTS
wind direction	°	DD
visibility	km	KMT

observedAt makes it temporal

The feed's valid_UTC value is 21.08.2026 10:00 UTC, which datetime does not read directly. The mapping changes the dots to slashes and removes UTC, producing the supported form 21/08/2026 10:00, parsed as UTC. That instant becomes the reading's observedAt.

An observedAt makes the entity temporal. The station ID stays stable, while the timestamp keeps successive readings distinct. The gold-price example uses the same approach for a monthly series; here each fetch adds a new observation for the station. The model also requires dateObserved, which comes from the same instant.

Meeting the model

WeatherObserved requires id, type, dateObserved, and location. The mapping supplies the timestamp and builds a point from the station's longitude and latitude. The model expects relativeHumidity as a ratio in [0, 1], while the feed reports a percentage, so the mapping divides it by 100. The result carries C62, the UN/CEFACT code for "one":

relativeHumidity: {
    source: "{{ metData.rh | int / 100 }}",
    type: "Property",
    transformation: "float",
    properties: {
        unitCode: {
            source: "C62",
        },
    },
},

The manifest

{
    version: "v1",
    inputs: [
        {
            source: "data/observation.xml",
            mapping: "weather.json5",
            format: "xml",
        },
    ],
    output: {
        target: "file",
        directory: "out",
        context: "none",
        validation: {
            mode: "fail-when-schema",
        },
    },
}

Run it

cassiopeia map \
    --manifest manifest.json5

The same run in a container mounts this directory at /data and makes it the working directory, so the paths do not change. For Podman, replace docker with podman and drop the --user line: rootless Podman already maps the container's root to your user.

docker run --rm \
    --user "$(id -u):$(id -g)" \
    --volume "$PWD:/data" \
    --volume "$HOME/.cache/cassiopeia-examples/schemas:/var/lib/cassiopeia/schemas" \
    --workdir /data \
    ghcr.io/vela-tools/cassiopeia:v1.0.2 \
    map \
    --manifest manifest.json5

From the repository root, the runner downloads the dataset, runs the mapping, and checks the output in one step:

cargo run -- run 14
cargo run -- run 14 --runtime docker

Read the result

The output contains one WeatherObserved entity. Each measurement carries its unit, and the temperature also carries the time it was read:

{
    "id": "urn:ngsi-ld:WeatherObserved:65142878",
    "type": "WeatherObserved",
    "stationName": {
        "type": "Property",
        "value": "LJUBLJANA/BEZIGRAD"
    },
    "dateObserved": {
        "type": "Property",
        "value": "2026-08-21T10:00:00.000Z"
    },
    "location": {
        "type": "GeoProperty",
        "value": {
            "type": "Point",
            "coordinates": [
                14.5172,
                46.0658
            ]
        }
    },
    "temperature": {
        "type": "Property",
        "value": 23.0,
        "observedAt": "2026-08-21T10:00:00Z",
        "unitCode": "CEL"
    },
    "relativeHumidity": {
        "type": "Property",
        "value": 0.8,
        "unitCode": "C62"
    },
    "atmosphericPressure": {
        "type": "Property",
        "value": 1011,
        "unitCode": "A97"
    },
    "windSpeed": {
        "type": "Property",
        "value": 0.2,
        "unitCode": "MTS"
    },
    "windDirection": {
        "type": "Property",
        "value": 54,
        "unitCode": "DD"
    },
    "visibility": {
        "type": "Property",
        "value": 20,
        "unitCode": "KMT"
    }
}

The entity passes the WeatherObserved schema. Its required fields are present, humidity is a ratio, and each reading states its unit and, for temperature, its observation time.