24.2.5. Georeferencer Plugin¶

Usual procedure

The Georeferencer Plugin is a tool for generating world files for rasters. It allows you to reference rasters to geographic or projected coordinate systems by creating a new GeoTiff or by adding a world file to the existing image. The basic approach to georeferencing a raster is to locate points on the raster for which you can accurately determine coordinates.

Features

アイコン	目的	アイコン	目的
	Open raster		Start georeferencing
	Generate GDAL Script		Load GCP Points
	Save GCP Points As		Transformation settings
	Add Point		Delete Point
	Move GCP Point		Pan
	Zoom In		Zoom Out
	Zoom To Layer		Zoom Last
	Zoom Next		Link Georeferencer to QGIS
	Link QGIS to Georeferencer		Full histogram stretch
	Local histogram stretch

Table Georeferencer: Georeferencer Tools

24.2.5.1. Usual procedure ¶

As X and Y coordinates (DMS (dd mm ss.ss), DD (dd.dd) or projected coordinates (mmmm.mm)), which correspond with the selected point on the image, two alternative procedures can be used:

The raster itself sometimes provides crosses with coordinates "written" on the image. In this case, you can enter the coordinates manually.
すでにジオリファレンスされたレイヤを使用します。これはベクタまたはラスタデータのいずれかで、ジオリファレンスしたい画像内にある同じオブジェクト/地物が含まれ、画像に使用したい投影のものです。この場合、QGIS地図キャンバスにロードされた参照データセットをクリックして座標を入力できます。

通常の画像ジオレファレンス手順では、ラスタ上の複数の点の選択、それらの座標指定、そして利用する変換タイプの選択が必要です。入力パラメータとデータに従ってプラグインではワールドファイルパラメータを計算します。座標を多く指定するほど結果はより良いものになるでしょう。

The first step is to start QGIS, load the Georeferencer Plugin (see プラグインダイアログ) and click on Raster ▶ Georeferencer, which appears in the QGIS menu bar. The Georeferencer Plugin dialog appears as shown in figure_georeferencer_dialog.

For this example, we are using a topo sheet of South Dakota from SDGS. It can later be visualized together with the data from the GRASS spearfish60 location. You can download the topo sheet here: https://grass.osgeo.org/sampledata/spearfish_toposheet.tar.gz.

図 24.19 Georeferencer Plugin Dialog¶

24.2.5.1.1. Entering ground control points (GCPs)¶

ジオリファレンスされていないラスタのジオリファレンスを開始するには、ボタンを使用してそのラスタをロードする必要があります。ラスタはダイアログのメインの作業領域に表示されます。ラスタがロードされると、基準点の入力を開始できます。
Using the ^{Add Point} button, add points to the main working area and enter their coordinates (see Figure figure_georeferencer_add_points). For this procedure you have three options:
- Click on a point in the raster image and enter the X and Y coordinates manually.
- 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.
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.

図 24.20 Add points to the raster image¶

通常一緒にラスター画像と地図に追加された点は、別個のテキストファイル（ [ファイル名].points ）に格納されます。これは、後日ジオリファレンサプラグインを再び開いて、新しいポイントを追加したり、結果を最適化するために、既存のものを削除できます。 mapX、mapY、pixelX、pixelY ：ポイント・ファイルは、フォームの値が含まれています。 ^{GCP点をロード} と ^{名前を付けてGCPポイントを保存} ボタンを使用してファイルを管理できます。

24.2.5.1.2. Defining the transformation settings ¶

After you have added your GCPs to the raster image, you need to define the transformation settings for the georeferencing process.

../../../../_images/transformation_settings.png

図 24.21 Defining the georeferencer transformation settings¶

24.2.5.1.2.1. Available Transformation algorithms ¶

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.
The Helmert transformation performs simple scaling and rotation transformations.
多項式 アルゴリズム1-3は最も広く使用されるアルゴリズムの一つで、変換元と変換先の地上基準点が一致するように導入されます。最も広く使用されている多項式アルゴリズムは二次多項式変換で、いくらかの湾曲が可能になります。一次多項式変換（アフィン）では共線性が保持され、縮尺変更、平行移動及び回転だけが可能になります。
The Thin Plate Spline (TPS) algorithm is a more modern georeferencing method, which is able to introduce local deformations in the data. This algorithm is useful when very low quality originals are being georeferenced.
The Projective transformation is a linear rotation and translation of coordinates.

24.2.5.1.2.2. Define the Resampling method ¶

The type of resampling you choose will likely depending on your input data and the ultimate objective of the exercise. If you don't want to change statistics of the image, you might want to choose 'Nearest neighbour', whereas a 'Cubic resampling' will likely provide a more smoothed result.

It is possible to choose between five different resampling methods:

Nearest neighbour
線形
キュービック
Cubic Spline
Lanczos

24.2.5.1.2.3. Define the transformation settings ¶

There are several options that need to be defined for the georeferenced output raster.

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 投影法の利用方法).
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.

24.2.5.1.3. Show and adapt raster properties ¶

Clicking on the Raster properties option in the Settings menu opens the Layer properties dialog of the raster file that you want to georeference.

24.2.5.1.4. Configure the georeferencer ¶

You can define whether you want to show GCP coordinates 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.

24.2.5.1.5. Running the transformation ¶

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