Lecture 8 – Geodatabase
create a geodatabase schema by
importing a data model
screen digitize simple features on top of
an aerial photo
understand proper digitizing techniques
create true curves and centerlines
determine a feature's location relative to
its direction or distance from another
feature
Data Model
A data model is a diagram that illustrates the design of a
geodatabase. Think of it as a storyboard of your
geodatabase's structure and content. It depicts the type
of data you require, the specific properties of the data
components, and the relationships between
components.
Screen digitizing
In addition to screen digitizing,
tablet digitizing is another
method of manually digitizing
features from a base layer.
Digitizing techniques
Choose a capture scale:
You should choose a view scale at which
you digitize features, or a capture scale,
that clearly portrays the features you want
to trace, and try to remain at the same
scale as you digitize. Saving your capture
scale in the metadata is a good idea.
Scale and Accuracy
A scanned 1:24,000 hardcopy map is your base layer, so displaying the
map at a scale of 1:24,000 should clearly show the features you want
to digitize. Although you may want to zoom in to a larger scale for
easier viewing, the accuracy of your new feature can never exceed the
accuracy of your base layer.
Curved features
When you digitize a curved line or polygon
segment in ArcGIS, you can create a true
curve as opposed to a series of small
straight line segments that represent a
curve, sometimes called a straight edge
curve. A true curve has only two vertices
as endpoints with an arc segment between
them.
A true curve has two vertices as endpoints with an arc segment
between them, instead of many vertices connecting straight line
segments. You can create tangent curves with ArcMap's Tangent tool
.
You can create non-tangent curves using ArcMap's Arc tool , which
creates a curve based on a specified point through which the curve passes,
or the End Point Arc tool , which creates a curve based on a user-defined
radius.
Midpoints
Perhaps you want to digitize a power pole between
two building corners, or create a street centerline
between parcels on opposite sides of the road. You
can easily create midpoints and centerlines using
the Midpoint tool .
To create a midpoint feature with the Midpoint tool,
you choose a pair of defining points, then the
Midpoint tool creates a new point feature in the
middle of them.
Centerlines
To create a centerline between two features with
the Midpoint tool, you choose multiple pairs of
defining points. The Midpoint tool creates
vertices in the middle of each pair, then
connects the vertices with a straight line.
Direction and distance
Sometimes the location of a feature is
described according to its direction and
distance from other features. In ArcMap,
you can create point features or vertices of
line features according to their direction or
distance from specified reference points.
Distance-Distance tool
The Distance-Distance
tool
creates a point
feature relative to two
reference points;
specifically, at one of
two intersections of two
circles around two
reference points. You
can choose at which of
the intersections the
new point or vertex will
be placed.
Direction-Distance tool
The Direction-Distance
tool
creates a point
feature or a vertex
relative to one or two
reference points. In
most cases, there will
be two points where the
circle and direction line
intersect; you then
choose one of the two
points where you want
to place the new line or
vertex.
To create a point or vertex with the
Direction-Distance tool, you specify a
direction angle from a reference point,
then specify a distance from the same
or a second reference point.
Snapping Properties
To digitize features in a precise location on
your base layer, you can use snapping to
make new features coincide with the
coordinates of existing features. Snapping
helps prevent gaps or overlaps between
features and speeds the digitizing process.
Refining Geometry
Sometimes features are digitized with more
vertices than are needed to adequately describe
them. Other times features are digitized with too
few vertices, giving them an unnatural or
inaccurate appearance.
ArcMap includes two tools for subtly reshaping
geometry. Both tools rely on a maximum
allowable offset (MAO), which is a buffer
distance that constrains the amount of change
allowed.
Generalize tool
The Generalize tool reduces the number
of vertices in a feature, thereby simplifying
its shape.
Digitized road Road vertices
MAO = 5‘
MAO = 10‘
MAO = 20'
In this example, the landform
details of the San Francisco Bay
Area are deemed important
because the map will be viewed
at different scales and spatial
analysis of the landform features
is anticipated.
Here, the same features of the San
Francisco Bay Area have been
generalized into more basic
geometry, which is acceptable for
use in location maps or maps that
will be printed at a smaller, fixed
scale
Smooth Tool
The Smooth tool
inserts additional vertices, thereby
creating a more natural appearance.
Digitized stream Stream vertices MAO = .5‘
MAO = 1'
MAO = 2'
Fixing overshoots and undershoots
Overshoots and undershoots are common
digitizing errors that may affect the quality
of your spatial data. Overshoots occur
when a line that is supposed to terminate
at the edge of another feature actually
extends past the edge. An undershoot
occurs when a line doesn’t reach the edge
where it is supposed to terminate.
Overshoot
To shorten an overshoot, use ArcMap's Trim
tool to trim it back to a selected edge.
Undershoot
To lengthen an undershoot, use ArcMap's
Extend tool
to extend it to a selected
edge.
Working with COGO
Coordinate Geometry (COGO) is used for
calculating the precise location and
configuration of features in your database
based on a traverse. A traverse is
represented by a series of consecutive lines
called courses. A course can be composed
of either a straight or curved line, and
typically has only two vertices (i.e., a twopoint line).
Traverse Tool
You can use the Traverse tool
or
to
translate survey field measurements into
geodatabase features by defining the start
point, which is a known point, and the
directions, distances, angles, and curves
of each traverse course.
In this example, a
traverse path is
described by
identifying a known
point, indicated by
the red triangle,
then identifying the
direction and
distance of each
subsequent line or
curved feature,
moving in a
clockwise
direction. The
traverse ends at
the beginning
point.