Plugins georeferencer

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2009年10月7日 (水) 04:34時点におけるArctic tern265 (トーク | 投稿記録)による版

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Georeferencer プラグイン

Georeferencerプラグインはラスタファイルに対するワールドファイルを生成するツールです。 このツールは、ワールドファイルを作成するか、新たな座標系へ変換を行うことによって、ラスタデータに対して測地座標もしくは投影座標を与えます。 ラスタデータに座標を与えるには、正確な座標がわかっているラスタデータ上の点を指定することが基本的な方法です。 座標値を与える情報源としては、以下のものが考えられます。

  1. ラスタデータそのもの。すなわち、ラスタデータに座標値が文字通り’記述されている’ことがあります。この場合はその値を手動で入力します。
  2. 座標値を持っているその他のデータ。すなわち、座標値を与えたいラスタデータにある目標物/地物のデータを持つラスタもしくはベクタデータです。この場合はQGISのマップキャンバスに読み込んだそれら参照データをクリックすることで座標を与えることができます。


まずはじめに、QGISを起動してGeoreferencerプラグインを読み込み(Section sec:load_core_plugin参照)、QGISツールバーメニューに表示されるGeoreferencer.png Georeferencerアイコンをクリックします。 すると図48のようにGeoreferencerプラグインのダイアログが表示されます。

ここでは例として、SDGS(訳注:South Dakota Geological Survey)のSouth Dakota州地形図を使用します。 この図は、この後でGRASS spearfish60 locationデータとともに表示することができるようになります。 地形図はこちらからダウンロードすることができます:

図 48: Georeferencer Plugin Dialog Nix.png
Entering ground control points (GCPs)

  1. To start georeferencing an unreferenced raster, we must load it using the ... browse 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 MActionCapturePoint.png Add Point button, add points to the main working area and enter their coordinates (See Figure 49). For this procedure you have two options:
    1. Click on a point in the raster map and enter the X and Y coordinates manually
    2. Click on a point in the raster map and choose the button Pencil.png from map canvas to add the X and Y coordinates with the help of a georeferenced map already loaded in QGIS.
  3. Continue entering points. You should have at least 4 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 49: Add points to the raster image Nix.png
Choose points.png

The points that are added to the map will be stored in a separate text file ([filename].points) which is stored 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 also Load GCPs and Save GCPs to different directories if you like.

Choosing the transformation

After you have added your GCPs to the raster image, you need to select the transformation type for the georeferencing process. Depending on how many ground control point 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 final result.

Currently, several algorithms are available:

  1. Linear
  2. Helmert
  3. Polynomial 1
  4. Polynomial 2
  5. Polynomial 3
  6. Thin plate spline (TPS)
  • 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.
  • The Polynomial algorithms are among the most widely used algorithms for georeferencing, and each one differs by the degree of distortion 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.
  • 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.
Running the transformation

  1. When the GCPs have been collected, and the transformation has been chosen, press either Create to create a new raster or Create and load layer to automatically add the new raster to the layer list.
  2. A warning message will appear that will inform you that a new raster (in GeoTIFF format) will be created.
  3. After hitting OK, you will also be asked to choose a resampling method. There are three methods available:
    1. Nearest neighbour
    2. Linear
    3. Cubic

Tip 44 Choosing 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.