GRASS GIS 8 Programmer's Manual
8.5.0dev(2024)-35ebcb33a3
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by GRASS Development Team (https://grass.osgeo.org)
Topology general characteristics:
Topology is written for native GRASS vector format; in case of linked OGR sources (see v.external
module), only pseudo-topology (boundaries constructed from polygons) is written.
The following rules apply to the vector data:
Vector map topology can be cleaned at user level by v.clean
command.
Topo file is read by Vect_open_topo().
Note: plus
is an instance of Plus_head data structure.
Name | Type | Number | Description |
plus->Version_Major | C | 1 | file version (major) |
plus->Version_Minor | C | 1 | file version (minor) |
plus->Back_Major | C | 1 | supported from GRASS version (major) |
plus->Back_Minor | C | 1 | supported from GRASS version (minor) |
plus->port->byte_order | C | 1 | little or big endian flag; files are written in machine native order but files in both little and big endian order may be readl; zero for little endian |
plus->head_size | L | 1 | header size |
plus->with_z | C | 1 | 2D or 3D flag; zero for 2D |
plus->box | D | 6 | Bounding box coordinates (N,S,E,W,T,B) |
plus->n_nodes, plus->n_lines, etc. | I | 7 | Number of nodes, edges, lines, areas, isles, volumes and holes |
plus->n_plines, plus->n_llines, etc. | I | 7 | Number of points, lines, boundaries, centroids, faces and kernels |
plus->Node_offset, plus->Edge_offset, etc. | L | 7 | Offset value for nodes, edges, lines, areas, isles, volumes and holes |
plus->coor_size | L | 1 | File size |
Nodes
For each node (plus->n_nodes):
Name | Type | Number | Description |
n_lines | I | 1 | Number of lines (0 for dead node) |
lines | I | n_lines | Line ids (negative id for line which ends at the node) |
angles | D | n_lines | Angle value |
n_edges | I | 1 | Reserved for edges (only for with_z ) |
x,y | D | 2 | Coordinate pair (2D) |
z | D | 1 | Only for with_z (3D) |
See P_node data structure.
Lines
For each line (plus->n_lines):
Name | Type | Number | Description |
feature type | C | 1 | 0 for dead line |
offset | L | 1 | Line offset |
N1 | I | 1 | Start node id (only if feature type is GV_LINE or GV_BOUNDARY) |
N2 | I | 1 | End node id (only if feature type is GV_LINE or GV_BOUNDARY) |
left | I | 1 | Left area id for feature type GV_BOUNDARY / Area id for feature type GV_CENTROID |
right | I | 1 | Right area id (for feature type GV_BOUNDARY) |
vol | I | 1 | Reserved for kernel (volume number, for feature type GV_KERNEL) |
See P_line data structure.
Areas
For each area (plus->n_areas):
Name | Type | Number | Description |
n_lines | I | 1 | number of boundaries |
lines | I | n_lines | Line ids forming exterior boundary (clockwise order, negative id for backward direction) |
n_isles | I | 1 | Number of isles |
isles | I | n_isles | Isle ids |
centroid | I | 1 | Centroid id |
See P_area data structure.
Isles
For each isle (plus->n_isle):
Name | Type | Number | Description |
n_lines | I | 1 | number of boundaries |
lines | I | n_lines | Line ids forming exterior boundary (counter-clockwise order, negative id for backward direction) |
area | I | 1 | Outer area id |
See P_isle data structure.
The vector library defines more topology levels (only for level of access 2):
Note: Only the geometry type GV_BOUNDARY is used to build areas. The geometry type GV_LINE cannot form an area.
Points
One point (nodes: 0, lines: 1, areas: 0, isles: 0) + L1
Line L1 (see P_line)
line = 1, type = 1 (GV_POINT)
Lines
One line (nodes: 2, lines: 1, areas: 0, isles: 0) +----L1----+ N1 N2
Node N1 (see P_node)
node = 1, n_lines = 1, xyz = 634624.746450, 223557.302231, 0.000000 line = 1, type = 2 (GV_LINE), angle = -0.436257
Node N2 (see P_node)
node = 2, n_lines = 1, xyz = 638677.484787, 221667.849899, 0.000000 line = -1, type = 2 (GV_LINE), angle = 2.705335
Line L1 (see P_line)
line = 1, type = 2 (GV_LINE), n1 = 1, n2 = 2
Areas without holes
Two lines (nodes: 1, lines: 2, areas: 1, isles: 1) +N1 / \ / \ / \ / +L2 \ / \ -------L1------
Node N1 (see P_node)
node = 1, n_lines = 2, xyz = 635720.081136, 225063.387424, 0.000000 line = 1, type = 4 (GV_BOUNDARY), angle = -2.245537 line = -1, type = 4 (GV_BOUNDARY), angle = -0.842926
Line L1 (see P_line)
line = 1, type = 4 (GV_BOUNDARY), n1 = 1, n2 = 1, left = 1, right = -1
Line L2 (see P_line)
line = 2, type = 8 (GV_CENTROID), area = 1
Area A1 (see P_area)
area = 1, n_lines = 1, n_isles = 0 centroid = 2 line = -1
Isle I1 (see P_isle)
isle = 1, n_lines = 1 area = 0 line = 1
Areas with holes
Three lines (nodes: 2, lines: 3, areas: 2, isles: 2) +N1 / \ / \ / \ / \ / +L2 \ / \ / +N2 \ / /\ \ / / \ \ / / \ \ / ---L3-- \ / \ ------------L1-------------
Node N1 (see P_node)
node = 1, n_lines = 2, xyz = 635720.081136, 225063.387424, 0.000000 line = 1, type = 4 (GV_BOUNDARY), angle = -2.245537 line = -1, type = 4 (GV_BOUNDARY), angle = -0.842926
Node N2 (see P_node)
node = 2, n_lines = 2, xyz = 636788.032454, 223173.935091, 0.000000 line = 3, type = 4 (GV_BOUNDARY), angle = -2.245537 line = -3, type = 4 (GV_BOUNDARY), angle = -0.866302
Line L1 (see P_line)
line = 1, type = 4 (GV_BOUNDARY), n1 = 1, n2 = 1, left = 1, right = -1
Line L2 (see P_line)
line = 2, type = 8 (GV_CENTROID), area = 1
Line L3 (see P_line)
line = 3, type = 4 (GV_BOUNDARY), n1 = 3, n2 = 3, left = 2, right = -2
Area A1 (see P_area)
area = 1, n_lines = 1, n_isles = 1 centroid = 2 line = -1 isle = 2
Area A2 (see P_area)
area = 2, n_lines = 1, n_isles = 0 centroid = 0 line = -3
Isle I1 (see P_isle)
isle = 1, n_lines = 1 area = 0 line = 1
Isle I2 (see P_isle)
isle = 2, n_lines = 1 area = 1 line = 3
Example 1
A polygon may be formed by many boundaries (several connected primitives). One boundary is shared by adjacent areas.
+--1--+--5--+ | | | 2 A 4 B 6 | | | +--3--+--7--+ 1,2,3,4,5,6,7 = 7 boundaries (primitives) A,B = 2 areas A+B = 1 isle
Example 2
This is handled correctly in GRASS: A can be filled, B filled differently.
+---------+ | A | +-----+ | | B | | +-----+ | | | +---------+ A, B = 2 areas A+B = 1 isle
In GRASS, whenever an 'inner' ring touches the boundary of an outside area, even in one point, it is no longer an 'inner' ring (isle in GRASS topology), it is simply another area. A, B above can never be exported from GRASS as polygon A with inner ring B because there are only 2 areas A and B and one island formed by A and B together.
Example 3
This is handled correctly in GRASS: Areas A1, A2, and A3 can be filled differently.
+---------------------+ | A1 | + +------+------+ | | | A2 | A3 | | + +------+------+ | | I1 | +---------------------+ A1,A2,A3 = 3 areas A1,A2+A3 = 2 isles
In GRASS, whenever an 'inner' ring does not touch the boundary of an outside area, also not in one point, it is an 'inner' ring (isle). The areas A2 and A3 form a single isle I1 located within area A1. The size of isle I1 is subtracted from the size of area A1 when calculating the size of area A1. Any centroids falling into isle I1 are excluded when searching for a centroid that can be attached to area A1. A1 above can be exported from GRASS as polygon A1 with inner ring I1.
Example 4
v.in.ogr/v.clean
can identify dangles and change the type from boundary to line (in TIGER data for example). Distinction between line and boundary isn't important only for dangles. Example:
+-----+-----+ | . | | . | +.....+.....+ | . | | x . | +-----+-----+ ---- road + boundary of one parcel => type boundary .... road => type line x parcel centroid (identifies whole area)
Because lines are not used to build areas, we have only one area/centroid, instead of 4 which would be necessary in TIGER.
Topology is generated for all kinds of vector types. Memory is not released by default. The programmer can force the library to release the memory by using Vect_set_release_support(). But: The programmer cannot run Vect_set_release_support() in mid process because all vectors are needed in the spatial index, which is needed to build topology.
Topology is also necessary for points in case of a vector network because the graph is built using topology information about lines and points.
The topology structure does not only store the topology but also the 'line' bounding box and line offset in coor file (index). The existing spatial index is using line ID in 'topology' structure to identify lines in 'coor' file. Currently it is not possible to build spatial index without topology.