Reproject Images



Reproject or transform raster image data from one map projection system to another using the Reproject Images dialog. You can also calibrate the existing input file to a new projection system that you specify, without generating a new file.

Comparing the reproject model of Geometric Correction to the reproject in the Raster tab > Geometry group, reproject in the Raster tab > Geometry group can directly transform the raster data without polynomial approximation when the fast approximation fails to achieve the geometric accuracy requirement. This is in cases where either the projection systems cannot easily be mapped with simple polynomials, or the geographic areas covered by the raster data are too large.

Reproject in the Raster tab > Geometry group picks a default cell size that preserves the input cell size. When converting between geographic and planimetric projections, a conversion factor of 1 degree to 110,000 meters is used. This approximation works well for most areas of the Earth's surface.

For information regarding conversion when using Reproject in the Geometric Correction model, see Set Geo Correction Input File.

Reprojection can be performed on a single georeferenced file or multiple georeferenced files using the Batch process.

Calculation Methods Improved for Certain Map Projections

Azimuthal Equidistant, Lamberts Azimuthal Equal Area, Orthographic, Robinson, or Sinusoidal

New spheroidal calculation methods are applied to these named projections starting with IMAGINE version 9.2 to correct some misalignment errors.

If you have misaligned data that was projected to one of these named projections prior to 9.2, simply reproject the data in IMAGINE version 9.3 or higher.

Dialog Description

The Reproject Images dialog opens when you:

Input File: Enter the name of the input file, or click the dropdown arrow to open the Recent Files menu.

open.png    Click to browse for the desired file.

Processing Option:    Select whether to calibrate the input file, or resample the input file to a new output file.

Calibrate Input File    Click to create a calibration record to add to the Input File. This saves the parameters of a Geometric Model to the Input File. When this option is selected, there is no output file created, thus only the Output Projection options are enabled.

For *.img files, the calibration is written directly to the image metadata.

For non-.img files, the calibration is written to a separate .aux file.

Resample to Output File    Click to reproject and resample the input file and save it as a new output file.

Output File: Enter the name for the output file.

open.png      In File Selector , choose the format type for the output file, and navigate to the directory to store the output file, or right-click to open the Recent Files menu.

Files of type    In the File Selector, click the dropdown arrow to select the output file format.

Output Projection: Projection categories and names can be defined by using Projection Chooser, which can be opened in several places throughout ERDAS IMAGINE.

Categories: Click the dropdown arrow to select the map projection category. This is the name of a library in which the map projection is saved.

Projection:    Click the dropdown arrow to select the name of the map projection.

globeClick this icon to create a new named map projection. The Projection Chooser dialog opens. Make your changes and then click the Save... button to save the new projection as a named item in a projection category. Once the category is saved, click OK on the Projection Chooser dialog to use the newly created projection.

Units:    Click the dropdown arrow to select the desired units of measure for the output map units. The units shown in the dropdown list depend on the Categories selection.

Ignore Zero in Stats.    When this checkbox is checked, pixels containing file values of zero (0) are omitted when statistics are calculated for the output file.

Output Cell Sizes:    You can choose to use the default cell size (same as the input image) or specify a different cell size for the Output File.

X:    Displays the default X Cell Size. If you wish to change the cell size, type a number or use the nudgers to select the cell size of output pixels in the X direction.

Y:    Displays the default Y Cell Size. If you wish to change the cell size, type a number or use the nudgers to select the cell size of output pixels in the Y direction.

Nominal... Click to enter the cell sizes as nominal meters. The Nominal Cell Sizes dialog opens.

Force Square Pixels on Reprojection    If you want square pixel size where X equals Y, check this option before projecting the image to another projected coordinate system. The default pixel size is rectangular where X and Y are not equal.

favorite    After you have determined your choice for Force Square Pixels on Reprojection, you can click this Set Preference icon to set the current setting as the default.

Snap pixel edges to    Click to turn on this option to snap the pixel edges of the output image to align to a raster image or to a point. Both options use pixel edge alignment.

raster image    To line up the edges of images such that there are no gaps or overlaps when the images are cropped, click this to snap pixel edges to a raster image that you specify.

File to snap to: (*.img)    Enter the name of the raster image, or click the dropdown arrow to open the Recent Files menu.

open.png     Click to open File Selector, or right-click to open the Recent Files menu.

a point    To line up pixel edges along grid lines, click this to snap pixel edges to a point that you specify. For example, if your output file has 10 meter pixel cell size (ground resolution), you can set the snap point to 0,0 to align all the pixels to every 10 meters in the map projection.

When you want to use pixel cell sizes (ground resolution) expressed in integers, rather than real numbers, use Snap to a point option. Enter the x,y coordinate of the output image that will be the origin of the grid to align the pixels to the grid. Then after processing, the pixels edges will line up along the grid where one whole pixel will equal the respective value of the grid.

X:    Enter the X coordinate of the point to snap to.

Y:    Enter the Y coordinate of the point to snap to.

When the map projection of the input file and the output file have different map orientations, or the input image is calibrated, the output file may contain unexpected results when applying Snap pixel edges option.

Set Output Corners Check this box to set the output corners for the input image.

ULX: Enter the upper-left X coordinate for the resampled image.

ULY: Enter the upper-left Y coordinate for the resampled image.

LRX: Enter the lower-right X coordinate for the resampled image.

LRY: Enter the lower-right Y coordinate for the resampled image.

The default output corners are computed from the input image and geometric model to resample the entire input image. This is done by transforming the four corners of the input image with the geometric model forward transformation. Note that these corners may not represent the entire input image space when using a nonaffine or nonfirst-order polynomial. The option to Recalculate Output Defaults samples a grid of points (rather than just the four corners) to more closely match the entire input image space.

From Inquire Box Click to define a subset area of the data by using the View Inquire Box. When you click this button, the coordinates are updated with the coordinates of the cursor box in the View.

To change these coordinates, you can move and/or resize the cursor box in the View, then click this button again.

The image you are using and the inquire box must already be displayed in a View in order to use the From Inquire Box option. Otherwise, you may manually enter coordinates from the projection system of the output image in the fields.

Do NOT use the input image (the one you are reprojecting) to get the subset coordinates. If you want to use the From Inquire Box option, make sure you are displaying an image that is in the same projection system of the output image you are going to. If not, when you open the From Inquire Box you will get coordinates that are not appropriate to the projection system you are reprojecting to.

Resample Method: Click the dropdown arrow to select a Resampling Method.

Nearest Neighbor   Assigns the value of the closest pixel.

Bilinear Interpolation    Uses the data file values of four pixels in a 2 x 2 window to calculate an output value with a bilinear function.

Cubic Convolution    Uses the data file values of 16 pixels in a 4 x 4 window to calculate an output value with a cubic function.

Bicubic Spline    Uses a block size of 5 x 5 or larger. This method fits a cubic spline surface through the current block of points. The output value is derived from the fitting surface that retains the values of the known points. This option may be available depending on which type of data is currently displayed or used in a process.

exclamation_point_icon The Bicubic Spline algorithm is much slower than other methods of interpolation.

LaGrange Uses the values of 16 pixels in a 4 x 4 window to calculate an output value with a cubic function that uses the LaGrange coefficients.

reference_iconSee Resampling Methods section in Producer Field Guide for more information.

Rigorous Transformation: Directly uses the original mathematical formula of projections for reprojection without approximation. This is a slow process, but the process has true geometric fidelity.

Polynomial Approximation: Uses polynomials to approximate the transformation between map projections. This is a fast and commonly accepted process. In terms of geometric accuracy, the approximation works for most raster images, especially when the geographic areas covered by the images are small. The root mean square error (RMS error) of the approximation is reported in the Session Log.

Maximum poly order: This is the maximum polynomial order allowed for polynomial approximation. When deriving the polynomial solution, the polynomials that have the lowest possible order and satisfy the tolerance are used.

Tolerance (pixels): This is the RMS error tolerance for polynomial approximation. Searching for the right polynomials is a sequential process starting with the first order of polynomials. The search stops once the tolerance is satisfied.

If tolerance exceeded: If the tolerance is not satisfied after searching from the first to the maximum polynomial order, choose one of these options:

Continue Approximation Continue to use polynomial approximation with the solution which has the lowest RMSE if the tolerance is exceeded.

Rigorous Transformation If the tolerance is exceeded, use direct transformation instead.

OK Click to perform this process and close the dialog.

Batch Open Batch Command Editor to schedule one or multiple processing jobs.

When batching the Reproject command, make sure that the projection name does not contain slashes. If it does, then it will be mistaken for a file name, which causes the Batch system to create the wrong variables.

Cancel Click to cancel the application and close the dialog.

Help Click to open this Help document.