GCP Installation

As discussed in Chapter 3, a theoretical minimum of three GCPs is necessary for adjusting an image to a reference coordinate system. In reality, more GCPs are necessary to cover an area with differing elevations well enough for georeferencing and to ensure that sufficient GCPs appear on each photograph (see Chapter 11.2). For more advanced photogrammetric analysis, a surplus of GCPs is advisable which can be used as check points and enable error assessment of digital elevation models (DEMs) derived from the images (Chandler, 1999). The points should be distributed evenly over the area so they represent the different terrain heights (Figs. 9-8 and 9-9A) and can be seen easily from above (i.e., not too close to trees, bushes, or walls so they are not concealed by shadows or objects tilted due to relief displacement). In order to avoid unnecessary work, it is also advisable to check during installation of the GCPs if they can be arranged so they are easily visible from one common vantage point for coordinate measurements (see below).

Especially when establishing permanent ground control for monitoring purposes, a detailed plan or sketch of the site and the control-point positions (Fig. 9-9B) is extremely valuable for avoiding GCP confusion during image processing and for re-discovering the points in the field in future missions. Although the images themselves would help to locate the GCPs, the first survey might not have covered the complete marked area and patterns on the ground might have changed, so additional information could be quite helpful, for example, "on highest point of small bump'' or "2 m south of dead pine tree.''

The position of the control point has to be fixed with some pinpoint object like a metal pole. Pieces of construction steel or water pipe are useful and cheap options and are easily available around the world from plumbing or hardware stores in the local industrial park. During the SFAP survey, the points have to be marked additionally with target signals for easy identification and accurate location in the image, preferably with eye-catching shapes and colors contrasting well with the natural background. If the ground cover is

FIGURE 9-9 Gully Negratin 3 near Baza, Granada Province, Spain. (A) Image map with ground control points. GCPs are distributed in and around the gully at varying terrain heights. Hot-air blimp aerial photograph by IM and JBR, March 2006. (B) Field-book sketch indicating the positions of GCPs and landmarks for orientation in the field. Note the internal distortions of the sketch map compared to the corresponding image map (A), which result from the difficulties of overseeing a complex terrain while mapping from different vantage points. Sketch taken from IM's field book 2001.

FIGURE 9-9 Gully Negratin 3 near Baza, Granada Province, Spain. (A) Image map with ground control points. GCPs are distributed in and around the gully at varying terrain heights. Hot-air blimp aerial photograph by IM and JBR, March 2006. (B) Field-book sketch indicating the positions of GCPs and landmarks for orientation in the field. Note the internal distortions of the sketch map compared to the corresponding image map (A), which result from the difficulties of overseeing a complex terrain while mapping from different vantage points. Sketch taken from IM's field book 2001.

uniform, e.g. grassland, and without features that allow an unambiguous allocation of the images, it might be necessary to use different markers in order to distinguish the individual points. In addition, a large north-arrow marker can be placed in proper position in the study site (see Fig. 2-3).

The size and design of the target signals influence their visibility in the digital image—they should be large enough to be identified easily in the image and yet enable the user to locate the actual point position accurately. Consider the example given in Chapter 2 with a vertical photograph acquired at a height of 100 m with 35 mm lens and 0.009 mm pixel size: the ground sample distance (GSD) is 2.6 cm. In order to be recognized as a distinct object, the target signal has to be at least 3-5 times larger than the GSD (8-13 cm). Depending on the "background" of the surface cover, this object could still be completely lost between patterns of vegetation, stones, and clumps of soil, so it would be best to surround this pinpoint signal centered over the GCP with something conspicuous, again 3-5 times larger, e.g. approximately 40 cm in diameter.

Depending on the nature of the study, temporary or permanent ground control could be desirable—for long-term monitoring, permanent control points that endure years or even decades are best, while for a one-time survey, a few hours for measuring the GCPs and taking the aerial photography might be enough. For long-term monitoring of various study sites in Spain, permanent GCPs were installed by hammering 40 cm (15-inch) long pieces of 2-cm metal water pipe into the ground so they protrude a few centimeters. A 30 cm x 30 cm red cardboard square with central hole is fitted over the pipe before the survey and secured by a long carpenter's nail on windy days. For improved definition of the point location, a white ring is painted around the edge of the pipe-hole (Fig. 9-10A). An even better definition of the exact location and height of the GCP (important for photogrammetric accuracy) could be achieved by fixing a circular target made from a CD and metal washer with a long nail and drop of glue to the pipe's top end (Fig. 9-10B). Both types of target make the GCP clearly definable in the aerial image (Fig. 9-11).

After the survey, the pipe should be covered with something protective and conspicuous like big stones to warn animals and people from stumbling and to make it easier to retrieve a year or two later. Keep in mind that it might be quite difficult to find something as small and inconspicuous as a piece of metal pipe unless well marked with something intentionally recognizable. When covered by vegetation or buried by soil, such markers also may be found again with a metal detector. However, the utilization of these devices is not allowed in all countries because they are often applied by tomb raiders and hobby archaeologists for illegal purposes. In Spain, for example, even carrying along a metal detector in a vehicle is indictable. The reader is therefore advised to seek information about the local regulations before employing a metal detector.

In many countries, metal pipes have a high value; they will be dug out by the local population and utilized for a more sensible purpose. Color-marked stones as well are eyecatching and seem too valuable to let them lie around in the field, so children tend to gather and carry them away. On a wasteland site in the periphery of El Houmer village in South Morocco, the installation of permanent GCPs seemed useless considering the continuing destruction by erosion and the many uses a piece of water pipe can have for the village youths. As a quick and easy method of GCP marking, red rings with 15-20 cm diameter were painted directly on the hard bare ground with spray paint (Fig. 9-12). Both SFAP and coordinate measurements were taken on the same

Photogrammetric Gcp Sample

FIGURE 9-10 Permanent GCPs made from a piece of metal water pipe. (A) GCP marked with a 30-cm cardboard target signal for the survey. (B) Top-precision GCP design. The exact top end of GCP is additionally marked with a circular target made from a CD labelled with target-rings. A metal washer is mounted in the center of the CD to hold a large nail inserted in the GCP pipe, and the CD is temporarily fixed with a few drops of glue to avoid displacement by wind. Photos by IM.

FIGURE 9-10 Permanent GCPs made from a piece of metal water pipe. (A) GCP marked with a 30-cm cardboard target signal for the survey. (B) Top-precision GCP design. The exact top end of GCP is additionally marked with a circular target made from a CD labelled with target-rings. A metal washer is mounted in the center of the CD to hold a large nail inserted in the GCP pipe, and the CD is temporarily fixed with a few drops of glue to avoid displacement by wind. Photos by IM.

FIGURE 9-11 The GCPs shown in Figure 9-10 appearing in aerial photographs with GSD ~3 cm. (A) GCP marked with red cardboard with white center ring. (B) GPC additionally marked with CD target. Kite aerial photographs (subsets, in original resolution) by IM and JBR, February 2009.

afternoon. Signals like these are quite sufficient if a local reference system for scaling and georeferencing photographs is needed in a one-time only survey.

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