Reprojecting files

When you add new data, it is reprojected automatically based on the data you already have open. When you open large, secondary files of a different projection, they are also reprojected automatically, which can slow the work. You might encounter this situation when, for example, a working file has a UTM projection and you open a file with an LCC projection. If you open the UTM data first, Focus must reproject the LCC to UTM. In such cases, it is recommended that you reproject the data manually and save it as a new file.

You can reproject both raster and vector data when the data is in a valid projection, it is in a GDB-supported format, and you know which datums were used. A datum is a mathematical surface used to make geographic computations. An ellipsoid defines the dimensions of the Earth. The datum includes the ellipsoid used and its position relative to the center of the Earth.

Each datum references one ellipsoid, but an ellipsoid can be referenced by one or more datums. If you compare the position of a point calculated using one datum, and then calculate the same position using a different datum, the coordinates of the point will differ, even if the datums refer to the same ellipsoid.

A datum is a mathematical surface used to make geographic computations and, therefore, it is possible to convert from one datum to another. However, reprojecting a file from one ellipsoid to another will not provide the correct results, because the critical information contained in a datum is not defined in the ellipsoid.