Create training data - orthoseg/orthoseg Wiki

General info

You can use any GIS tool you want to create the training data, as long as it can edit .gpkg files. I personally use QGIS on a windows computer, but even though in some cases I might give specific instructions based on the environment I use, I'm sure valid alternatives exist using any other GIS tool on eg. linux.

Before you start digitizing examples, it is a good idea to put a definition of the subject you want to segment on paper, with a clear scope of things you want to include or want to exclude. While you are busy creating your training dataset this might still evolve based on examples you encounter and didn't think of at this stage, but as it is important that the training dataset is consequent, make sure the definition is/stays clear.

The orthoseg training data is (by default) organized as follows. In your {project_dir} there is a directory called "labels". In this directory there should be 2 files per image layer that you want to use to base your training on:

  • {segment_subject}_{image_layer_name}_locations.gpkg
  • {segment_subject}_{image_layer_name}_polygons.gpkg

In the "locations" file you need to draw the bounding boxes of the images that will be downloaded and used as the images to train the detection on. In the "polygons" file you need to digitize the things you want to detect as polygons as shown in the screenshot below. The green square is the "location", the red rectangle is the "polygon" drawn to show where the football field is in the "location". This training example will help the neural network to learn how a (part of a) football field looks, but also that tennis fields are not to be detected...

Digitize training data, location and polygon

Creating training data is -in my experience- an iterative process. You add some samples to the training dataset, you evaluate the results, add some more,... This way you can minimize the number of training samples you need to digitize/train on which can save a lot of time.

So, the following steps need to be iterated over till you are happy with the results:

Search for good examples and add them to the training input

There are different strategies possible to find good examples to add to the training input:

1. On random locations

For the initial examples, you can use an existing dataset to find relevant locations or just zoom to random locations distributed over the territory you want to segment.

2. Use the vectorized result of a prediction on the entire territory

If a run has been done on the entire territory, a vectorized file of the segmentation will be available. Using this, there are several options to look for new examples using it:

  1. Do overlays with existing datasets that can help to find missing examples.
  2. Check locations in a random way distributed on the territory to find segmentation errors.
  3. In the vector file of the segmentation result, a column "nbcoords" will be available. Order this column descending to find the polygons with the most points: often these polygons with many points are bad segmentations.

3. Use the result of a previous training run

If you already did a training run, it is often useful to look for errors in the training data. After training, orthoseg will automatically run an "evaluation" prediction on all the images in the training, validation and test datasets. This won't be saved as a vector file, but these predictions will be saved as images in the following directories:

  • {project_dir}/training/{traindata_version}/train/{segment_subject}_..._eval
  • {project_dir}/training/{traindata_version}/validation/{segment_subject}_..._eval
  • {project_dir}/training/{traindata_version}/test/{segment_subject}_..._eval

Each of these directories is useful to look at in the following way:

  • If you open the directory with the predictions and set the view in eg. "Windows explorer" to "large tiles", you will see the original image, followed by the mask and by the prediction, so you can easily compare them.
  • The file names of the predictions are formatted this way: <prediction_quality>_<x_min>_<y_min>_<x_max>_<y_max>_<nb_pix_x>_<nb_pix_y>_<image_type>.tif
    • <prediction_quality>: is the percentage overlap between the mask (as digitized in the training dataset) and the prediction, so examples with problems will have a small percentage overlap and will be shown first in the directory.
    • <x_min>,...: these are the coordinates of the location of the image. So if you want to correct a digitization of an image in eg. QGIS, you can:
      1. copy/paste the <x_min>_<y_min> to the "coordinate" field the status bar (below) in QGIS
      2. replace the "_" by ","
      3. press the ENTER key, and you'll be in the right location.
    • <image_type>: 'mask' for the digitized mask, 'pred' for the prediction,...

4. Add good examples found to the training dataset

The way you digitize the examples in the training dataset will influence the quality of the result significantly, so here is some advice:

  • Digitize the examples for the training process as precise as you want the result to be.
  • If you need relatively complex interpretation yourself to deduct things from the image, the AI probably won't succeed in doing it... yet. An example of something I didn't get good results with:
    • A road that goes into a forest and comes out of it a bit further might be logic to a human that it will probably continue under the trees, but this didn't work in my quick tests.
  • If you want detailed results, make sure you digitize the examples on the same ortho imagery you are going to train the network on. Certainly for subjects that have a certain height (eg. buildings, trees,...) a significant translation of eg. the roof can occur between different acquisitions that can influence the accuracy.
  • With the default parameters, the training works on images of 512x512 pixels and the prediction on 2304x2304. So the AI won't be able to take into account more information than available on those images. So if you need to zoom out and look on larger area's than those, take in account the AI won't have this option.

If you find good examples, you can add them to the training dataset as such:

  1. First, add the location that will be used to determine the extent of the image to the {subject}_{image_layer_name}_locations file. The bounding box of the image generated for the training process will be the minx and miny of the bounding box of the polygon digitized. The width and height of the image are determined by the pixel width and pixel height as defined in the configuration files. Par example, using the default pixel width and height of 512 pixels, and a pixel size of 0.25 meter/pixel the image width and height will be 128 meter (512 pixels * 0.25 meter/pixel). Ideally you try to digitize the location polygon as close to the size as the generated image will be, this way it is clear for which area you need to digitize the exact labeled polygons in the next step. The Advanced digitizing panel in QGIS makes this quite trivial. You need to fill out the following properties for each location:
    • traindata_type (text): determines how the image generated based on this location will be used in the training. Possible values: 'train', 'validation', 'test', 'todo'.
      • locations set to 'train' will actually be used to train the AI on.
      • locations set to 'validation' will be used during the training to crosscheck if the AI is sufficiently learning in a general way so it will be working well on data it has never seen before instead of specializing on the specific cases in the 'train' locations. The best results are obtained if 10% to 20% of the locations are set to 'validation' and try to focus on as many different common examples as possible, but evade including difficult situations in the 'validation', only set those to 'train'.
      • optionally, you can set locations to 'test': they won't be used in the training process, but after the training run the segmentation will be applied to them, including reporting. Eg. if you have an existing dataset with examples of moderate quality or if you encounter situations you are not sure about, you can set them to 'test'.
      • optionally, you can set locations to 'todo': they won't be used at all, but this class is practical to signify that the location still needs work: eg. the polygons still need to be digitized.
    • description (text): optional: a description of the label location Remark:
    • train images can't/won't overlap: when looping through the locations digitized, if an image location overlaps with an already generated image, it will be skipped.
  2. If you are adding a 'false positive', so a location where the segmentation thinks the subject is present on this location, but it isn't, you are already ready and can look for another example.
  3. If you are adding a 'false negative', or any new example, now you need to digitize the actual subject in the {subject}_labeldata file. It is important to digitize all samples of the subject in the location area added in the previous step, as any surface that isn't digitized will be treated (and trained) as a 'false positive'. The following properties need to be filled out:
    • label_name (text): the content of the label data digitized. The label names you can use are defined in the subjects configuration file. If it is different than the ones specified there, it will be ignored.
    • description (text): optional: a description of the feature digitized.

Run training + (optionally) a (full) prediction

Instructions to run a train, predict,... session can be found here.

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