Data Management: Coverages
ARC commands for coverage maintenance: creating, maintaining, modifying
Generation: generate
Topology Maintenance: build, clean
Coordinate Adjustment: projectdefine, project, transform, Adjust
Registration: register, rectify
Joining: mapjoin, Append, matchnode
Modifying: Dissolve, Clip, densifyarc, generalize
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Overview (repeat)
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•
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The manipulation of coverages lies at the heart of ArcInfo-based GIS
The methods or commands are numerous and can be classified in many ways
There is always overlap, but basic groupings are
– Coverage Query: techniques for querying existing coverages (see query.ppt)
– Coverage Maintenance: creating, maintaining and modifying coverages (see covergages.ppt)
– Coverage Conversion: converting from one GIS format or data model to another (see
convert.ppt)
– Spatial Analysis: manipulation of coverages to answer some research question or achieve
some operational purpose (see analysis.ppt)
• Spatial analysis techniques may be used for coverage query anmd maintenace also
•
We will discuss Query and Maintenance in the context of Workstation ArcInfo.
– query.ppt
– coverage.ppt
ArcToolbox includes these in its Data Management component
•
We will cover the Conversion and Spatial Analysis commands in the context of
ArcToolbox,
– Conversion.ppt
– Analysis.ppt
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Spatial Analysis
Coverage Maintenance
(in this lecture)
• Coverage Generation (ARC)
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•
register
rectify
Joining coverages (ARC)
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–
•
projectdefine
project
transform
Adjust
Registration
–
–
•
build
clean
Coordinate Adjustment (ARC)
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–
–
–
•
generate
Topology Maintenance (ARC)
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–
•
Some of the spatial analysis methods,
especially polygon overlay, can also be
useful for coverage “maintenance”. In
essence, you use one coverage to control
or effect changes in another. These will
be covered in a later lecture.
mapjoin
append and matchnode
Modifying coverages (ARC)
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–
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–
•
Polygon Overlay (ARC)
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clip
split
erase
update
union
intersect
identity
no attribute merging
with attribute merging
Dissolve
Clip (also in polygon overlay)
densifyarc
generalize
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Coverage Generation
• You can generate a new coverage from any comma-delimited ascii file
that contains a unique identification number (will become the <cover>id variable, or user-id) and coordinate pairs for points, lines, or
polygons.
• Prior to creating a coverage, determine if you would like single
precision (coordinate accuracy to 3 decimals for geographic decimal
degree) or double precision (coordinate accuracy to 6 decimals)
• EXAMPLE FOR POINTS:
– Arc:
precision double
– Arc:
generate airports
– Generate: input airports.gen
– Generate: points
– Generate: quit
– Arc:
Remember to build topology after creation of a new coverage
– Arc:
build airports point
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Constructing Topology
• BUILD - creates or updates topology and the feature attribute table for
a coverage
– BUILD <cover> {POLY | LINE | POINT | NODE | ANNO.<subclass>}
– build airports point
• CLEAN - generates a coverage with correct polygon or arc-node
topology. To do this, CLEAN edits and corrects geometric coordinate
errors, assembles arcs into polygons and creates feature attribute
information for each polygon or arc (i.e., creates a PAT or AAT).
– CLEAN <in_cover> {out_cover} {dangle_length} {fuzzy_tolerance}
{POLY | LINE}
– (See topology in Lecture 1: Arc/INFO Basics)
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Adjusting Coverage Coordinates
There are three basic commands that adjust coverage coordinate values:
PROJECT, ADJUST, and TRANSFORM
• PROJECT changes the coordinates from one map projection to another.
PROJECT is the most mathematically accurate.
• ADJUST and TRANSFORM use corrections that you’ve specified through
either links or tics.
– ADJUST - adjusts or “rubbersheets” a coverage’s features in either direction along
the links (control points) in a separate link coverage or file. Links represent the
from/to locations used in the adjustment. Adjust will localize coordinate changes.
– TRANSFORM - uses tics common to the coverage whose coordinates are to be
transformed and an existing coverage to change the coordinates to match the output
coverage. Transform scales, skews, rotates, and shifts all coordinates in a coverage
but it does not perform ‘rubber sheeting’. Often used to transform a coverage from
digitizer units to real-world coordinates.
For georeferencing an image, use REGISTER and RECTIFY
• REGISTER—interactive application which performs homogeneous
transformation for an image
• RECTIFY is required after REGISTER to convert the registered image from
image coordinates to map coordinates.
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Changing Projections
• Before you begin you must know:
Current Projection of coverage
Desired Projection of coverage
• FIVE COMPONENTS OF A PROJECTION
Coordinate System
US ALBERS, LAMBERT
Datum
NAD27, NAD83
Spheroid
CLARK 1866, WGS84, GRS90
Units
DD, METERS, FEET
Parameters
Depends on Coordinate System
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Before you can project a coverage/grid or file to a new
projection, you must know the current projection.
Where would you find this information?
• “Describe” the coverage to see if projection shown
• Data Source -- metadata
• Best Guess--Trial and error
Two commands to work with projections
projectdefine -- define the projection
project - actually projects the coverage from one coordinate
system to another
After you project data, do a BUILD on the file.
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Arc: describe rain
Description of SINGLE precision coverage rain
FEATURE CLASSES
Feature Class
------------ARCS
POLYGONS
Subclass
--------
Number of Attribute
Spatial
Features data(bytes) Index?
Topology?
--------- ------------ ------- --------239
36
111
28
Yes
SECONDARY FEATURES
Tics
Arc Segments
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13462
TOLERANCES
Fuzzy
=
0.303 V
Dangle
=
0.001 V
COVERAGE BOUNDARY
Xmin =
Ymin =
2255128.700
6866000.000
Xmax =
Ymax =
2622000.000
7098000.000
STATUS
The coverage has not been Edited since the last BUILD or CLEAN.
NO COORDINATE SYSTEM DEFINED
No projection
information available
Defining the Projection
• If the projection is not defined (not displayed with the
‘describe’ command) you may elect to define the projection
prior to projecting it to another coordinate system.
• PROJECTDEFINE prompts an interactive dialog for
entering the projection information for a data set.
• Usage: PROJECTDEFINE <COVER | GRID | FILE | TIN> <target>
• Arc: projectdefine cover trails
Define Projection:
Project: projection geographic
Project: units dd
Project: datum NAD27
Project: parameters
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Second Example
Arc: projectdefine cover texas
Define Projection
Project: projection Albers
Project: datum NAD27
Project: units meters
Project: spheroid clarke1866
Project: parameters
1st standard parallel [ 0 0 0.000]: 29 30 0.0
2nd standard parallel [ 0 0 0.000]: 45 30 0.0
Central meridian [ 0 0 0.000]: -96 0 0.0
Latitude of projections origin [ 0 0 0.000]: 23 0 0.0
False easting (meters): [ 0.000]: 0.0
False northing (meters): [ 0.000]: 0.0
Project: end
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PROJECTDEFINE & PROJECTCOPY
• All projections, whether or not they have parameters, must have the
word PARAMETERS entered to complete the projection definition.
• PROJECTDEFINE will not change the coordinates of the <target>
dataset. To project a dataset from one projection to another, you must
use the Arc PROJECT command.
• For coverages, tins and grids, the projection information is stored as a
PRJ file within their subdirectory. The projection information for a file
is stored as filename.prj.
• The absence of a prj file results in the projection type being
UNKNOWN.
• PROJECTCOPY allows you to copy the projection information from
another coverage (essentially, it copies the .prj file) to save you
entering all the info required by PROJECTDEFINE
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Projecting a Coverage
PROJECT converts a geographic data set between two coordinate
systems. PROJECT prompts an interactive dialog for entering the
projection information for a data set.
Usage: PROJECT <COVER | GRID | FILE> <input>
<output> {projection_file}
Arc: project cover trails trails_sp
- will need to enter details at prompt
OR
Arc: project cover trails trails_sp dd_state83.prj
- requires a projection file called dd_state83.prj
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Sample without a projection file,
with input projection defined:
Arc: project cover trails trails_sp
*************************************************
The INPUT projection has been defined
*************************************************
Use OUTPUT to define the output projection and END to finish
Project:
Project:
Project:
Project:
Project:
Project:
Project:
OUTPUT
projection stateplane
zone 5351
datum NAD83
units feet
parameters
end
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Sample without a projection file,
with input projection not defined:
Arc: project cover trails trails_sp
Use INPUT to define the input projection, OUTPUT to define the output
projection, and END to finish
Project:
Project:
Project:
Project:
Project:
Project:
Project:
Project:
Project:
Project:
Project:
Project:
INPUT
projection geographic
units dd
datum NAD27
parameters
OUTPUT
projection stateplane
zone 5351
datum NAD83
units feet
parameters
end
The parameters command must be included even if there are none to enter.
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Projection File - input projection not defined
Any type of text file, saved with extension .prj
Input
projection albers
datum NAD27
spheroid clarke1866
units meters
parameters
29 30 0.0
45 30 0.0
-96 0 0.0
23 0 0.0
0.0
0.0
output
projection stateplane
zone 5351
units feet
datum nad83
spheroid grs1980
parameters
end
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Georeferencing an Image
• REGISTER initiates an interactive program that allows you to
georeference an image. The program provides views of the image and
coverage data, and a menu from which the georeferencing operations
are performed. From these windows, a series of links, or displacement
vectors, are added that join image locations to map coordinates.
• Register identifies an appropriate transformation. This is saved in a
"tiff world file" (.tfw) which is added to your workspace
(note: it's a homogeneous transformation, not a rubber sheeting)
arc: register plano.tif streets
• RECTIFY is used to convert the registered image from image
coordinates to map coordinates. RECTIFY creates a new rotated,
scaled and transformed image based upon the parameters in the world
file. You must use rectify to apply this transformation permanently before it
can be used in Arc/Info. This is not necessary to view it in ArcView
arc: rectify plano.tif planorec.tif
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DRAWING IMAGES
•
•
•
•
•
The Arcplot command IMAGE draws a raster image or a group of images
from an image catalog.
IMAGE <image | image_catalog> {band}
<image | image_catalog> - name of an image or image catalog. The image
must be in a format supported by the IMAGE INTEGRATOR as shown in the
following table.
{band} - an integer value indicating the band to be displayed; the default band
is 1.
Image catalogs are INFO data files that store the pathname and spatial extent
of each image in the catalog.
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IMAGES SUPPORTED
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Image format
Image naming convention
TIFF
no suffix required
Sun Raster file
no suffix required
JPEG
no suffix required
Run-length Compressed (RLC) image.rlc
ERDAS
image.gisimage.lan
IMAGINE
image.img
Band Interleaved by Line (BIL) image.bil
Band Interleaved by Pixel (BIP) image.bip
Band Sequential (BSQ)
image.bsq
GRASS
Use GRASS naming conventions
Arc Digitized Raster Graphics Use ADRG naming conventions
Windows bitmap (BMP)
image.bmp
JFIF (JPEG compression)
image.jpg
Grid
no suffix; grids are directories
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Joining Adjacent Coverages
• Data in separate but adjacent map sheets may be joined into a larger,
seamless, coverage.
• There are two commands to merge adjacent coverages into one.
• The APPEND command is used for coverages with either point
topology or arc topology.
• MAPJOIN is recommended for coverages with polygon topology, or
both polygon and arc topology.
• Generally, if MAPJOIN won’t work, try APPEND
• Useful for files which come by county, for example TIGER files
• Files and their INFO tables must be clean and consistent across
boundary:
– May have to do ArcEdit on each first
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APPEND
• APPEND combines up to 500 coverages into one coverage.
• APPEND <out_cover> {NOTEST | template_cover | feature_class...
feature_class} {NONE | FEATURES | TICS | ALL}
• {NOTEST} - Feature attribute tables are not appended.
• {NONE | FEATURES | TICS | ALL} - specifies how tics and coverage
features will be numbered in the <out_cover>.
– NONE - neither Tic-IDs nor feature User-IDs will be modified. This is the
default option.
– TICS - ID offsets will be calculated for tics.
– FEATURES - User-ID offsets will be calculated for the feature class(es)
specified by the <feature_classes> argument. Tic-IDs are not modified.
– ALL - ID offsets will be calculated for both tics and features.
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MAPJOIN
• MAPJOIN - combines up to 500 adjacent coverages containing polygon
or networked features into one coverage and recreates topology.
– Mapjoin combines the APPEND and CLEAN processes
Usage: MAPJOIN <out_cover> {feature_class...feature_class | template_cover}
{NONE | FEATURES | TICS | ALL} {clip_cover}
Arc: mapjoin soil poly features studyarea
Enter coverages to be MAPJOINed (Type END or a blank line when done):
=========================================================
Enter the first coverage: soil1
Enter the second coverage: soil2
Enter the third coverage: <cr>
Done entering coverage names (Y/N)? y
Do you wish to use the above files (Y/N)? y
MAPJOINing files…
Arc:
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Seamless Merging of Polygon Coverages
• One method to merge two (or more) geographically adjacent polygon
coverages into one involves preparing coverages for edgematching,
edgematch the coverages in ArcEdit, MAPJOIN the coverages, and then
DISSOLVE the edge feature.
• Step 1: Prepare adjacent coverages for edgematching
– Remove coverage data overlap with SPLIT, CLIP or ERASE • Step 2: EDGEMATCH - Invokes an edge matching process in ArcEdit
• Step 3: Check item definitions and attribute values -– definitions of items to the right of cover-id must match exactly, and
attribute values must correspond (i.e. if soil type is 33 in polys that will
join, the value must be 33 in both tables).
• Step 4: MAPJOIN in Arc - will be prompted for coverages to join
• Step 5: DISSOLVE the boundaries between the mapjoined coverages
to create a seamless cover - aggregate based on attribute values
• Step 6: Reconstruct topology (BUILD or CLEAN)
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Merging of Point or Arc Coverages
•
One method to merge two (or more) geographically adjacent point or line
coverages into one seamless coverage involves the commands APPEND and
MATCHNODE (arcs only)
•
•
•
•
Step 1: Visually compare spatial data - edit, if needed.
Step 2: Make sure item definitions and attribute values match
Step 3: Perform APPEND - will be prompted for coverages to join
Step 4: Perform MATCHNODE to snap nodes to nodes or nodes to
points, and to extend dangling arcs to intersect other arcs.
Step 5: Reconstruct topology
•
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COVERAGE OPERATION: DISSOLVE
•
•
•
DISSOLVE - eliminates arcs between adjacent polygons that contain equal
values for a specified item.
DISSOLVE <in_cover> <out_cover> <dissolve_item | #ALL> {POLY | LINE
| NET | REGION.subclass}
You can specify any item from the <in_cover> to be the <dissolve_item>.
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Dissolve, pg 2
– Can be used to create a new polygon coverage based on variables
contained within the file. For example, the Tiger/LINE file from the
census bureau contains data for right and left polygons for each arc. From
this file polygon topology can be built, and then items on which dissolve
is possible includes census tract, census block, census block group, zip
code, traffic analysis zones.
– While the input coverage may contain information concerning many
feature attributes, the output coverage contains information only about the
dissolve item.
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CLIP
•
Strictly speaking, this is a Polygon Overlay spatial analysis command (and its
covered in more detail in Analysis), but its commonly used in coverage
maintenance, basically to limit the extent of one coverage to the area desired
– E.g. you have a highway map for Texas which you want to limit to D/FW only
•
CLIP - extracts those features from an input coverage that overlap with a clip
coverage. It extracts a portion of a coverage to create a new coverage.
CLIP <in_cover> <clip_cover> <out_cover> {POLY | LINE | POINT | NET | LINK |
RAW} {fuzzy_tolerance}
Three covers specified: in, clip and out.
The <clip_cover> must have polygon topology.
Only the outermost boundaries in the <clip_cover> are used; interior polygons are ignored.
CLIP uses the clip coverage as a cookie cutter; only those input coverage features that are
within the clip coverage are stored in the output coverage.
Topology is built for the output coverage.
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DENSIFYARC
• DENSIFYARC - adds vertices to arcs in a coverage at a specified
interval and alternately splits the arcs at each new vertex.
– DENSIFYARC <in_cover> {out_cover} <interval> {VERTEX | ARC}
– <interval> - the distance in coverage units which will separate new vertices.
Existing vertices will be kept for each arc. DENSIFYARC will add new vertices to
each arc by starting at the from node, traveling the <interval> along the arc, adding a
vertex, and so on. This process continues until the end of the arc is reached. Then
the next arc is densified.
– {VERTEX | ARC} - this specifies if vertices are added at each <interval> or arcs are
split at each interval.
– The vertex option will generate one arc with 5 vertices
– The arc option generates 6 new arcs with no vertices.
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GENERALIZE
•
GENERALIZE - reduces detail within coverage arcs a specified line
simplification operator and tolerance.
– GENERALIZE <in_cover> <out_cover> <weed_tolerance> {POINTREMOVE |
BENDSIMPLIFY}
• <weed_tolerance> sets the tolerance in coverage units used to remove unwanted detail
within arcs.
• {POINTREMOVE | BENDSIMPLIFY} - specifies the line simplification operator.
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FUZZY TOLERANCE
• {fuzzy_tolerance} - nodes that are within the
fuzzy_tolerance distance are merged into a clean junction.
before
after
fuzzy tolerance
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FUZZY TOLERANCE, pg 2
• Determining fuzzy tolerance if not given in the command line:
– 1) Tolerance values are read from the existing coverage TOL file.
– 2) If no TOL file exists and the width of the BND is between 1 and 100,
the tolerance is 0.002.
– 3) Otherwise, the tolerance is 1/10,000 of the width or height of the BND,
whichever is greater.
(BND is the file containing the extent of the coverage, thus BND is
essentially the map width or height in map units.)
• A fuzzy tolerance value of 0 will not be accepted by most operations.
• Common fuzzy tolerances range from .001 to 1.
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