Treatment of Vector Data in SPRING
This page presents some definitions about the vector structure and
the procedures for the edition of maps of the type thematic, cadastral,
networks, and DTM through digitalization.
The edition of vector data in SPRING is made on maps of the type
thematic, cadastral, networks, and digital terrain model (DTM).
The geographic objects have intrinsic properties like location
(position of an object in geographic coordinates), dimensions (allows
the description of the object by entities like points, lines, or
areas), continuity, size, neighborhood, form, and scale that must
be known in order to be represented.
The maps in SPRING can have their representation in the vector and
raster formats, depending on the category of the data (see about the conversion of representations).
However, in this page we will only deal with the edition of vector
data.
For a thematic, cadastral, network, or numeric map we use
elements like points, lines, and areas (or polygons), to define the
thematic classes, geographic objects and numeric samples (contour lines
and
elevation points)
See also:
Editing Maps in SPRING.
Raster Edition.
Elements of vector structure
Vector data edition in SPRING is performed on thematic, cadastral, network, and DTM (digital terrain model) maps.
The vector representation of these maps is the most precise way of
representing a geographic object.
In terms of computational implementation the storage and retrieval
of this format is more complex than on the case of the raster or bitmap
representation. In SPRING it has been implemented a structure of
"v-r-trees" to facilitate the access to the data.
Point
A point is defined as any
geographic entity that can be located by a pair of coordinates (x,y).
Points are used to represent the location of a geographic phenomenon in a place, or
to represent some feature in a map that is too small to be shown as an
area or line. Examples: location of a city, an airport runway, the
summit of a mountain or an elevation point (when it has an attribute Z,
besides the X, Y coordinates, that could be the elevation or some
other parameter). The figure below shows a point P as defined by a
cartesian system.

Line
A line is an entity
defined by at least two pairs of
(x,y) coordinates, that is, two points.
Lines are used to represent map features that are
too thin to be shown as an area or that theoretically have no width.
Examples: a river, a road, the coast line of a continent, a contour
line or an administrative limit.

When a line has some attribute Z, besides the (x,y)
coordinates of the points that constitute it, it is called an Contour line In an Contour line every one of
its points has the same value of Z. An Contour line is only defined in ILs
of numeric category.
Areas
Areas are defined as a series of (x,y) coordinates forming
line segments that close in a polygon.
Frequently in geographic information systems, areas
are represented by polygons. Examples: the geographic extension of a
city, a lake, a deforested area.

See also:
How to obtain conceptual
information and tips
Vector Edition
Vector Representation in SPRING
In SPRING the three elements above are translated into geographic
features that are represented by: nodes,
points, arcs, contour lines, islands, polygonal lines, and polygons.
Arc
An arc is
a set of points interconnected by line segments that start and end
at node.

Examples of arcs.
Arcs are used to model the frontiers of the
polygons. This way they are
used to delimit objects that define areas.
A node is a special type
of point whose objective is to define the intersection point of two or more arcs.
The figure below illustrates a polygon formed by arcs and nodes. Two
adjacent polygons can share one same arc, provided the intersection of
the lines be delimited by the presence of a node.
In the following example there are two polygons,
the first formed by arcs 1 and 2, and the second by arcs 2, and 3, that
could represent rice and soybean areas.

Points
Points are entities used to represent features that
are too small (2D points) to be represented by a polygon or to
represent a numeric sample (3D point). A 2D point is normally
associated to a symbol in thematic, cadastral, and network maps defined
in the database, besides its non-spatial attributes. For example: a
church or a lighting pole that cannot be represented in the present
scale.

Islands
Islands are a special type of
polygon delimited by one arc only. Only one node defines the initial
and the end point of the polygon, since there are no adjacent polygons.
The figure below shows an arc that starts and ends in the same node,
thus defining an island.

Polygonal Line
Polygonal Line or open polygon is formed by a set of
points interconnected by line segments that begin and end at a node.The
difference between a polygonal line and an arc resides in the fact that
a polygonal line never defines an area (polygon). It is used to model
linear features like, for instance, lines that represent geological
fractures, rivers, roads, and other geographic elements that could be
observed as linear features in the adopted scale.

The polygonal line is
used when the intersection point of the lines should not be modeled
and so there is the need of introducing a node.
Contour Line
A contour line
can be seen as a polygonal line where one
only value of Z is given. The figure below shows two contour
lines with different heights. More details about contour lines and
their usage can be found in Numeric Modeling.

See
also:
How to obtain
conceptual information and tips.
Vector Editing
Topology
Points, lines, and polygons are vector representations normally used
to describe geographic objects in maps. The spatial relationship among
these entities, like for example, proximity and neighborhood, is
obtained by the interpreter from the analysis and observation of the
maps.
However, once the objects in the map have been digitized and are
represented by points, lines, and polygons in the computer, such
spatial relationship should be explicitly defined so that the spatial
data analysis operations can be performed.
In digital maps the spatial
relationships are described by the topology defined as the mathematical
subject that studies the geometrical properties that do not vary with a
deformation. Shapes and object coordinates are less important than the
elements of the topological model like: connectedness, compactness,
continuity, etc...

A simplified form of an example of topological structure that is
generated for a thematic map.
This way, defining topology means making clear the spatial
relationships among the objects by means of a mathematical process.
Defining the topology of data of the thematic or cadastral model in
SPPRING results in the creation of polygons,
that is, the system will store the information about the lines, nodes
and identifiers that compose each polygon, as well as about the lines
that are shared by different polygons (including the neighborhood and circunscrividade
(?) among them).
In SPRING the topology, as
related to nodes and the neighborhood of arcs, can be automatically
defined during digitization. When digitizing a line, a node will be
automatically inserted when intercepting other line or at its ends.
However, the creation of the polygons must be made so that the whole
topology can be defined for the Infolayer.
Once the topology is defined, each polygon can then
be associated to a thematic class,
or to an object of the cadastral map, or yet, to a network segment,
provided they are defined in the Database.
See
also:
The Conceptual Scheme of SPRING
Vector Editing
Vector Editing
In the process of vector editing in SPRING, specially in cadastral,
thematic, and network maps, the user has to go through the steps
of Digitizing, Adjustments, and Polygonalization. For the edition of
a numeric IL one only needs the Digitizing
and some eventual adjustments.
Digitizing
Digitizing is a process
that allows the conversion of spatial data from analog to digital
media. Digitally these data are structured in such a way as to allow
the execution of operations that are typical of spatial analysis.
The user may digitize the lines by introducing them point after
point, in the Pass mode, or
simply by following the feature with the mouse continually activated,
in the Continuous mode. In
this mode you can also define the frequency that the points will be
acquired to compose the lines via the Digitizing
Step. This parameter corresponds to the interval between the
points of the digitized line. This parameter is given in millimeters in
the scale of the Infolayer being edited. Check the scale of the IL in Edit - Infolayer
in the main menu. Remember that the cartographic precision of maps is
in the order of 0.3 mm times the map scale. So a digitizing step that
is less than this value will be out of the precision limit of the map
itself.
In SPRING the digitization of data can be performed with the
definition of Automatic or Manual Topology. In automatic topology each time an arc
intercepts another a node will be automatically defined, without the
need to have the operator indicating it. This mode is ideal for the
digitization of polygons, being valid only for the line being digitized.
In manual topology the
introduction of nodes or line breaks should be performed by the
operator. This mode is indicated, for example, for the digitization of
geologic fracture lines, where one line should remain integral even of
others cross it.
The digitization an be performed via different peripherals like
the digitizing table, (the most common), image scanning devices or a
video monitor (screen).
SPRING allows the digitization of data by the table or by the
video monitor as described below.
- Digitization with the table
The digitizing table is basically constituted of two parts:
- a plane surface, electronically sensitive, where the map or
graphic to be digitized is placed;
- a mouse, magnetic device , that sends the (x,y)
coordinates of a point on the table to the computer.
l
The digitizing table mouse has the function of acquiring the (x,y)
coordinates that will be related to the geographic coordinates. This is
done through the buttons that play specific roles for each desired
objective.
By activating the data acquisition from the digitizing table
mouse, the compute mouse is deactivated.
In the manual digitization of data, the buttons of the table mouse
have pre-established functions.
- Button 1
has the function of point acquisition, that is, it can edit points and
lines. Corresponds to the select
button of the computer mouse.
- Button 2
sends to the system the information of end of line. Corresponds to the adjust button of the computer mouse.
- Button 3
has no special function, and so is not used.
- Button 4
has the function of finalizing the manual digitization of the data via
table. When pressed, the table mouse becomes inactive, and the
controlling functions get back to the computer mouse.
Before digitizing via the table, we must perform its calibration.
Firmly fix the map over the table, in such a way as to allow no
clearances. You must verify that the map lies within the sensitive
limits of the table, as shown in the figure above.
- Digitizing with the screen
The user can digitize the lines and points of the map on the
screen itself, using the computer mouse for the definition of the
geographic objects, according to the description of the mouse buttons:
- The select button or
left button of the mouse makes the line editing; point addition, point
movement, depending on the operation selected in the edit window.
- The menu button or right button defines the end of a
line or arc (by inserting a second node). Can be used to show all the
points and nodes of a line, independently of the editing function that
is selected.
Errors associated to vector
digitization
Following we present some errors for the guidance
of the SPRING users during digitization.
User has digitized an insufficient
number of points: the representation of curved formats depends
on the number of vertices used. Consequently, the errors related to the
digitization of straight lines is much smaller than the ones obtained
by the digitization of complex curves. Example:

The definition of a suitable value for the Digitizing Step can minimize this
error. We warn the user, though, to the fact that too small steps will
generate lines with too many points.
Since some errors can be
avoided and others provoked by the choice of the topology type (manual
or automatic), the errors will presented for each topology type.
Manual Topology
A- The user did not define a node
- in a polygon, every line that intercepts another line should have a
node associated to the intersection point, for example:

In this case, we must insert a point in the line that was
intercepted and transform it into a node (operation break line) and
only after join the lines or adjust them automatically.
B-) The user did not make the
super positioning of the nodes: in the digitization the polygon
is kept open, or a line does not reach or exceeds the interception
point. Example:

For these cases the automatic adjustment of the nodes could
not be sufficient to close these polygons, one should then perform the
manual edition, joining or approximating the lines, or even increasing
the threshold of automatic adjustment.
Automatic Topology
A-) The user exceeded the limit of the intersection -
since a line can be automatically broken, a short line could be left
over and should thus be manually eliminated (operation delete line), in
case the digitizing step is smaller than the left over line length.
Such error demands the operator attention during digitization.

B-) The user did not define correctly the limits
between polygons: in the digitalization the lines can be
superimposed or you may leave a gap between them, when a message of
invalid incorporation is presented.
Superimposition - since
nodes were inserted the error will only be detected during the polygon
generation, and should be fixed through the manual edition.
Gap - this error cannot be detected by
the editing operations of the system, and is related to the precision
of the operator during digitization.

Adjustments
The adjustment step consists in making the arcs having their ends,
that is nodes, connected. Usually we use the automatic adjustment when
the errors are small or are within the tolerance limits defined by the
user. When greater errors are inserted it becomes necessary to perform
the manual adjustments ( see more
details about the adjustment tools).
Polygonalization
Once all the lines were adjusted, we must create the topology by
performing the polygonalization step, when the topological relationship
between the polygons will be created or updated. See more in how to digitize.
See also:
How to calibrate the Digitizing
Table
How to digitize a Map ?
How to Create an IL for
Digitizing
?
Vector Editing
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