Reproject – Resample – Realign Operation
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The Reproject operation outputs an MVR virtual raster that is a copy, or a partial copy, of an input source with a different coordinate system to the input source. An input source can be a raster source or a raster file. If the raster source contains multiple raster files, then they are merged. Reproject supports batch processing where multiple raster inputs can be processed serially.
The Resample operation outputs an MVR virtual raster that is a copy, or a partial copy, of an input source with a different base resolution cell size to the input source. An input source can be a raster source or a raster file. If the raster source contains multiple raster files, then they are merged. Reproject supports batch processing where multiple raster inputs can be processed serially.
The Realign operation outputs an MVR virtual raster that is a copy, or a partial copy, of an input source with a different base resolution cell alignment to the input source. An input source can be a raster source or a raster file. If the raster source contains multiple raster files, then they are merged. Reproject supports batch processing where multiple raster inputs can be processed serially.
The Reproject, Resample, and Realign operations all operate on the same principle and use the same dialog, although not all controls are enabled in all operations.
Reproject allows you to change the coordinate system of a raster. When you do, it is likely that you will also want to change the cell size (resample) and change the cell edge coordinates (realign). The dialog allows you to make all these changes at once.
The current coordinate system of the raster is shown on the dialog, and an information button is provided to provide details about this coordinate system. You can choose any recently used coordinate system by selecting from the new coordinate system drop-list. Alternatively, hit the Projection Explorer button to browse to a new coordinate system.
It is likely that the change of coordinate system will trigger a change in cell size. For example, if the current coordinate system is geographic and the cell size is expressed in degrees and the new coordinate system is UTM, then you will want an equivalent cell size in metres. Because of the nature of MVR rasters, it is not necessary to get this cell size conversion perfectly correct. In fact, all that the cell size really determines is the size of the raster at the base resolution. This is the resolution level at which you will likely compute statistics or export to a different format, so it is important. However, it is not critical to rendering or other processing operations.
You can enter the cell size as a decimal value (width in X and height in Y) or as a fraction. In the latter case you specify a signed integer numerator and an unsigned integer denominator. For example, a cell size of one arc-second is (1/3600) degrees. If possible, it is a good idea to express the cell size as a fraction. The raster engine uses fractional mathematics to eliminate or minimise round-off errors in floating point numbers.
If you change the cell size, it is likely that you will want to specify the cell alignment. You do this by providing X and Y coordinates for the tile origin. Just like cell sizes, you can specify these coordinates as decimal values or as fractions, and fractions are preferred.
When you move the tile origin for a virtual raster, you do not actually relocate the raster data. All you are doing is specifying where the bottom left corner of cell (0,0) is at base resolution. The tile coordinate system is infinite, so you can put it anywhere. However, you will usually place it at the bottom left corner of the raster.
Moving the tile origin does change the coordinates on which the cell boundaries lie. For example, if you have a cell size that is 100x100m and you place the tile origin at (0,0) then the cell edges will lie on values that follow the series 0, 100, 200,… etc and the cell centre will lie on 50, 150,250,… etc. If you want the cell centre to lie on the 0, 100,200,… values then move the origin to (50,50).
When you change the coordinate system, cell size or cell alignment, it will trigger interpolation in the virtual raster engine. At the end of the day, the engine will try to populate a raster mesh with the best possible data from a source raster. It will do this by targeting a resolution level in the source raster that is the same or better resolution than the destination mesh. It will interpolate data from this level using nearest neighbour, bi-linear, or bi-cubic methods depending on the field type, band data type, and user preferences.
If you choose the Resample operation, the coordinate system information will be shown but not editable. The cell size and tile origin controls will be editable. If you choose the Realign operation, only the tile origin controls will be editable.