4.3 Spatial schema (ISO 19107)
137
GM_Placement and GM_AffinePlacement
Placements are meant to take a standard geometric construction such as a circle,
and place it as a symbol in a geographic dataset. In most cases a placement involves
a shift of dimension from the 2D-space to 3D-space.
The placement (class GM_Placement) allows any type of geometric transformation to be applied during the output to the target system. With the transformation involved, a circle may be changed to any type of conic.
The affine placement (class GM_AffinePlacement) allows a linear transformation
only (6 parameters).
A clothoid (class GM _ Clothoid) is a plane curve whose curvature is a fixed function of its length. The following equation is valid at any position along the clothoid:
length * radius = constant.
The clothoid is used to layout roads and railway lines.
y
straight line
Fig. 4.9. GM _ Clothoid (Larisch 2003)
GM_OffsetCurve
x
An off set curve (class GM _ OffsetCurve) is a curve at a constant distance from the
basis curve. They can be useful as a cheap and simple alternative to constructing
curves that are off sets by definition.
GM_SplineCurve
Spline curves (class GM _ SplineCurve) may be polynomial splines or B-splines.