Image Coordinates Ground Coordinates

2. Background for Image Coordinate Transformation Services

This topic volume is the portion of the OpenGIS™ Abstract Specification that covers image coordinate conversion services. That is, this part of the abstract specification describes services for transforming image position coordinates to and from ground position coordinates. These services might alternately be called “Image Geometry Model Services.” This section provides background information useful in understanding the Image Coordinate Transformation Services discussed in later sections.

2.1. Image Coordinates

The position of a point in an image is specified in two-dimensional 2-D image coordinates. A point of interest may be either a real or virtual point. The material in this topic volume is currently limited to 2-D images; it could be expanded in the future to include 3-D images. For a digital image, the 2-D image coordinates are usually specified in the row and column directions of image pixels. The reference point for these image coordinates is usually the corner pixel of the image where the pixel row and column indices are either 1, 1 or 0, 0. Alternately, the reference point for image coordinates could be the center pixel, when the number of pixels is an odd number in each axis. When the number of pixels is even in each axis, the reference point for image coordinates could be the middle of the four most-center pixels. Image coordinates could be specified as integers, referring to pixel indices. However, image coordinates are usually specified as floating point numbers, to allow coordinates to represent positions with fractional pixel spacings. The center of the corner pixel is often considered to have coordinates 1, 1. Alternately, the outside corner of the corner pixel could be considered to have coordinates 0, 0. For a film or hardcopy image, the 2-D image coordinates are usually specified in two orthogonal axes, which could be called x and y or u and v. Various reference points can be used, such as the “center” of the image. These image coordinates are often specified as floating point numbers, in units of millimeters.

2.2. Ground Coordinates

The position of a point in ground space is specified in three-dimensional 3-D coordinates. This document uses the term ground coordinates; photogrammetry often uses the alternate term “object coordinates.” Again, a point of interest may be a real or virtual point. For Image Coordinate Transformation Services, ground coordinates are almost always 3-D, since the position of a point in all three dimensions affects the image position corresponding to that point. These ground coordinates are specified in some Spatial Reference System SRS, such as any SRS defined in Abstract Specification Topic 2.

2.3. Position Accuracy