Setting the projection

A projection is a method of portraying all or part of the Earth on a flat surface. The output projection defines the final projection for orthoimages, mosaics, three-dimensional (3-D) features, and digital elevation models (DEM). The GCP projection is the default used when manually collecting ground control points (GCP) to specify the projection of the collected GCPs or when importing GCPs from a text file.

When you collect GCPs from a geocoded source, the coordinates are reprojected to the GCP projection and written to the project file. When you collect GCPs from multiple sources, in the Set Projection window, you can change the GCP projection to match each source. Using different projections increases processing time during orthorectification; therefore, working with one projection is more efficient.

The elevation reference for GCPs must match the elevation reference of the imagery in your project. When you set the projection, the elevation datum is set by default. If the source of your GCPs differs from that set in your project, OrthoEngine can automatically convert the values for your project.

Elevation values can be referenced to a number of different surfaces, but for mapping operations, you usually use elevations above mean sea level (MSL), which is based on a geoid. With most math models, the model is based on GCPs that are also based on the geoid.

There are two math models that are based on orbital information instead of GCPs: the radar-specific model and the rational-functions model when it is used with the IKONOS GEO Ortho Kit product. Orbit information is always referenced to an ellipsoid, and the ellipsoid number is taken from the projection information defined in the file.

To convert between ellipsoid and MSL, OrthoEngine calculates the difference between the geoid and the ellipsoid at the point in question, and then applies the difference to compute the transformed elevation.

Note: You need not set the projection; that is, in the Set Projection window, click Cancel to close the window. The output projection and resolution will be set automatically from the first image you add to your project.

The Set Projection window may be displayed after completing the Project Information window.

To set the projection

Under Output Projection, in the box to the right of Earth Model, type the projection string.
For example, type UTM 17 T D000.

If you do not know the the projection string:
1. In the list to the left of the Earth Model button, click a projection type.
  When you select UTM, SPCS (State Plane Coordinate System), or Other, additional windows may appear in which you select the parameters to define the projection. You can also click More to open the window relevant to your selection. Select the parameters you want, and then click Accept.
2. Click Earth Model.
3. Click either the Datum or the Ellipsoids tab.
4. Click the datum or ellipsoid you want, and then click Accept.
If you are creating a project to mosaic raw images and you do not have ground control, select Pixel as your output projection.
If your project type supports GCPs, complete steps 4 to 6, as applicable; otherwise, proceed to step 7.
In the Output pixel spacing box, type the pixel x-size according to the units (meters, feet, degrees, or pixels) in the project.
In the Output line spacing box, type the pixel y-size according to the units (meters, feet, degrees, or pixels) in the project.
With orthoimages and mosaics, the values of Output pixel spacing and Output line spacing are the x and y resolution of your output images.
Under GCP Projection, do one of the following:
- If you know the projection string, type it in the box to the right of Earth Model.
  For example, type UTM 17 T D000.
- If your GCP projections are the same as your output projection, click Set GCP Projection based on Output Projection.
If you do not know the projection string, follow the same steps as you used to set the output projection.
In the Elevation unit list, click an appropriate unit of measure for the GCP elevations, as applicable.
If necessary, in the Elevation reference list, click a vertical reference for the elevation values of any GCPs.
Click OK.