Модуль привязки растров¶

Ввод контрольных точек¶

1. To start georeferencing an unreferenced raster, we must load it using the button. The raster will show up in the main working area of the dialog. Once the raster is loaded, we can start to enter reference points.
2. Using the ^{Add Point} button, add points to the main working area and enter their coordinates (see Figure figure_georeferencer_2). For this procedure you have three options:

   • Щелкнуть мышью по точке на растровом изображении и ввести координаты X и Y вручную.

   • Click on a point in the raster image and choose the ^{From map canvas} button to add the X and Y coordinates with the help of a georeferenced map already loaded in the QGIS map canvas.
   • With the button, you can move the GCPs in both windows, if they are at the wrong place.
3. Continue entering points. You should have at least four points, and the more coordinates you can provide, the better the result will be. There are additional tools on the plugin dialog to zoom and pan the working area in order to locate a relevant set of GCP points.

Figure Georeferencer 2:

Добавление точек на растр

The points that are added to the map will be stored in a separate text file ([filename].points) usually together with the raster image. This allows us to reopen the Georeferencer plugin at a later date and add new points or delete existing ones to optimize the result. The points file contains values of the form: mapX, mapY, pixelX, pixelY. You can use the ^{Load GCP points} and ^{Save GCP points as} buttons to manage the files.

Определение параметров трансформации¶

После того, как контрольные точки добавлены на растровое изображение, необходимо определить параметры преобразования для привязки.

Figure Georeferencer 3:

Определение параметров трансформации модуля привязки

Доступные алгоритмы преобразования¶

Depending on how many ground control points you have captured, you may want to use different transformation algorithms. Choice of transformation algorithm is also dependent on the type and quality of input data and the amount of geometric distortion that you are willing to introduce to the final result.

Currently, the following Transformation types are available:

• The Linear algorithm is used to create a world file and is different from the other algorithms, as it does not actually transform the raster. This algorithm likely won’t be sufficient if you are dealing with scanned material.

• Трансформация Хельмерта совершает простые трансформации с изменением масштаба и вращением.

• The Polynomial algorithms 1-3 are among the most widely used algorithms introduced to match source and destination ground control points. The most widely used polynomial algorithm is the second-order polynomial transformation, which allows some curvature. First-order polynomial transformation (affine) preserves colliniarity and allows scaling, translation and rotation only.

• Алгоритм тонкостенного сплайна — более современный метод привязки, дающий возможность ввода в данные местных деформаций. Данный алгоритм очень полезен, когда необходимо привязать растры с низким качеством изображения.

• Проективная трансформация — линейное вращение и сдвиг растра.

Определение метода пересчёта¶

Выбранный тип пересчёта будет, скорее всего, зависеть от исходных данных и конкретной цели операции. Если вы не желаете менять совокупную информацию изображения, вам, возможно, подойдет метод «ближайший сосед», тогда как «кубический» пересчет приведет к более сглаженному результату.

It is possible to choose between five different resampling methods:

1. Ближайший сосед

2. Линейный

3. Кубический

4. Кубический сплайн

5. Ланцоша

Установка параметров трансформации¶

Существует несколько параметров, которые необходимо задать для получения привязанного растра.

• The Create world file checkbox is only available if you decide to use the linear transformation type, because this means that the raster image actually won’t be transformed. In this case, the Output raster field is not activated, because only a new world file will be created.
• For all other transformation types, you have to define an Output raster. As default, a new file ([filename]_modified) will be created in the same folder together with the original raster image.
• As a next step, you have to define the Target SRS (Spatial Reference System) for the georeferenced raster (see Работа с проекциями).
• If you like, you can generate a pdf map and also a pdf report. The report includes information about the used transformation parameters, an image of the residuals and a list with all GCPs and their RMS errors.
• Furthermore, you can activate the Set Target Resolution checkbox and define the pixel resolution of the output raster. Default horizontal and vertical resolution is 1.
• The Use 0 for transparency when needed can be activated, if pixels with the value 0 shall be visualized transparent. In our example toposheet, all white areas would be transparent.
• Finally, Load in QGIS when done loads the output raster automatically into the QGIS map canvas when the transformation is done.

Просмотр и изменение свойств растра¶

Выбор пунта Свойства растра в меню Параметры вызовет диалог свойств привязываемого слоя.

Настройки модуля¶

• You can define whether you want to show GCP coordiniates and/or IDs.
• As residual units, pixels and map units can be chosen.
• For the PDF report, a left and right margin can be defined and you can also set the paper size for the PDF map.
• Finally, you can activate to Show Georeferencer window docked.

Запуск преобразования¶

After all GCPs have been collected and all transformation settings are defined, just press the ^{Start georeferencing} button to create the new georeferenced raster.