Create QField plugin for generating flightplans, including user configuration fields & DEM sampling

[spwoodcock]

Requirement

• Rather than develop a mobile app ourselves, we could use a QField plugin.
• QML plugins can use JavaScript, so we would need to convert the login in the drone-flightplan package to JS.

User Story

1. Go to QField project, with plugin already synced.
2. Click on the task area square on the map --> the plugin button 'Generate flightplan' appears.
3. User clicks button to generate flightplan, and the file is generated and downloaded to their phone.
4. (bonus) instead of downloading the file, the user already has the drone controller plugged into their phone via USB. Clicking 'Generate flightplan' copies the file to the relevant place on the external storage of the controller, making it immediately flyable!

Implementation

DEM raster pixel value extraction

• One key barrier is the elevation extraction from the DEM layer - can we do this within the plugin?
• The DEM is cut out from a global DEM using our AOI, then will be included in the QField project files.
• Alternatively we could make a web query for this, but (1) I'm not sure how feasible it is (2) it's less robust than bundling a <50MB DEM file.

Flightplan generation in JavaScript

• We also need to call the .js logic within the QML, return the geojson, and load it as a layer into the project.
• The actual logic for this is mostly Python, with some basic geospatial algorithms, e.g. generating a grid of squares over an area. So we need to convert to JavaScript and QGIS engine (geospatial queries) logic for this.
• I hope we can easily hook into the QGIS engine for this.

Getting the flightplan file

• Ideally we can also convert the geojson --> WPML XML, then download onto the users device to loading on the drone.
• We could just download this file to the users device, then they follow the usual method of copying the file to the connected drone flight controller filesystem (Android).
• Alternatively, it looks like we have WRITE_EXTERNAL_STORAGE permission for QField already: https://github.com/opengisch/QField/blob/master/platform/android/AndroidManifest.xml.in#L32, so is anything stopping us doing this directly from the plugin, as long as the drone controller is plugged into the phone via USB?

WIP at https://github.com/hotosm/drone-tm/tree/develop/src/qfield-plugin

Additional considerations

• If the QML plugin setup doesn't work for our needs, do we have other options?
  1. Extend the plugin capabilities in QField core via contribution?
  2. Write a plugin in C++? Does QField support this?
• It may actually be possible to write to external storage using: https://github.com/opengisch/QField/blob/cc510873c4aab441f9d36812b8724b2196b9e01f/src/core/utils/fileutils.h#L95 (however looks like this will only work on Android, not iOS).

[linear]

TECH-793

[nirvn]

One key barrier is the elevation extraction from the DEM layer - can we do this within the plugin?

You could do this via an ExpressionEvaluator item, e.g.:

  ExpressionEvaluator {
    id: expressionEvaluator
    feature: feature // feature as you iterate through features
    layer: vectorLayer // point layer from which you iterate through features
    project: qgisProject 
    
    expressionText: "raster_value('dem_layer', 1, @geometry)" // the @geometry is taken from the expression evaluator's feature property
  }

To evaluate the expression:

  function getDEMValue() {
    const value = expressionEvaluator.evaluate()
  }

Now, that's a lot of effort to get to the value :) we could also add an invokable function in the LayerUtils class to fetch that value directly through javascript too. I'd be happy to guide you through adding that functionality.

Alternatively we could make a web query for this, but (1) I'm not sure how feasible it is (2) it's less robust than bundling a <50MB DEM file.

Agreed, a web query is not optimal, and would force you to be online to generate flightplans to begin with.