Transcript Document

Layering, buffering,
registering, digitizing…
Peter Fox
GIS for Science
ERTH 4750 (98271)
Week 3, Tuesday, February 7, 2012
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Contents
• Reading review
• Assignment 1
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Layering
Buffering
Registering
Digitizing
Lab on Friday
Next class(es)
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Reading review for last week
• MapInfo User Guide and other docs
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Layering
• Useful for:
– Separating contents, structures and developing
themes
• Implies distinction from other layers??
• MapInfo (and many others) has
– A Cosmetic layer
– Turn on/ off detail option
• AKA Coverage or Level
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Layer examples
USGS: (L) Shaded-relief map and contour lines generated from
the digital elevation model in the study area. (R) The wetlands in
the study area ranked according to their vulnerability to pollution
on the basis of combination of factors evaluated by GIS
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Add a layer
• Be as clear as you can about the layer ‘type’
– Points, lines, polygons/ regions
– Theme based (e.g. in MapInfo – represented by
icons), range, chart, special symbol, grid,
raster…
– Note also ‘Group Layers’
• Layer Control (from Main menu or Map
menu)
• Practice on Friday!
• User guide, through p.68
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Change layer properties
• Labels (style, placement), lines, nodes,
centroids, etc.
• For Cosmetic layer – has restrictions so make
sure you read (MapInfo Help system):
– Using the Cosmetic Layer
– Saving Cosmetic Layer Objects
– Saving Objects on the Cosmetic Layer
– Removing Cosmetic Objects
– Disabling the Save Cosmetic Objects Warning
Dialog Box
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Reorder layers
• Map layers display in the order that they are
listed in the Layer Control window, with the
bottom layer drawn first and the top layer
(always the Cosmetic Layer) drawn last
• It is important to order your layers correctly
and thus give them some thought or be
prepared to experiment!
• Hint: Denser information sources typical go
on the bottom, lighter ones on top…
• Something for Friday – learn how to change
the layer order
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Layers to improve visibility
• E.g. zoom layering – DC street maps, you set
the threshold, ~ 5 miles but this depends on
your ‘use’
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Combining layers
• The process of combining and transforming
information from different layers is sometimes
called map "algebra" insofar as it involves
adding and subtracting information (intersect,
identity, union, symmetric diff., etc.
• Try this on Friday - e.g. open a grid layer, see
p.77 (open one of the files downloaded from
the /gis/data directory)
• Create a new layer of derivative or
complementary information
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• Combine and save…
Buffering
• Buffer zone - an area within a given ‘distance’
from a map feature
• Points, lines, or polygons can be buffered
• Buffers are used to identify areas surrounding
geographic features
– E.g. to keep septic systems over 100 meters
away from streams, locate housing within a
quarter mile of existing roads, keep hiking trails
away from seasonally flooded rivers, or make
sure most of your city is within some maximum
distance from a fire station or school
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What is produced?
• When you buffer on a set of features, the
output is a set of polygons.
• Buffering points or lines creates a new
coverage that is a polygon coverage.
• Polygons define an inside and outside region
– Inside region = an area less than the specified
buffer distance from the feature(s) of interest
– Outside region = an area more than the specified
buffer distance from the feature(s) of interest
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Buffering example
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Distances - ~ two types
• Fixed – as it implies
– E.g. in the two previous examples
– Storage of distance is simpler
• Variable – buffer distance depends on
another attribute of the ‘feature’ (entity)
– E.g. in the next example, based on area value,
but could be distance from some other entity, or
land type, or statutes, or …
– Storage of distances is more complex
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Variable buffer - area
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Variable buffer – intrinsic property
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Inside/ Outside – used for?
• These inside and outside regions are typically
distinguished by different codes in an
attribute table
• You should know the specific codes assigned
for the software system you use
– Why? You may want to label them, color them,
establish relations between/ among them, etc.
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Registering
• Registration consists of specifying the
coordinates of a minimum number of points
(3) on the image to orient it in geographic
space – they are called control points
• Three points are needed to determine the
axes and scale
• You should also try to determine the type of
projection used in the original map
• Note, this is different from geocoding (we’ll do
that next week)
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Where from?
• Everywhere!
• A raster image is a type of computerized
image that consists of row after row of tiny
dots (pixels) – see last Friday’s class
• E.g. you can create a raster image by
scanning a paper map
• Make sure you store the image in a file that is
a supported format
• Then, display the file using MapInfo
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Formats
• There are many different raster image file
formats
• MapInfo Professional can read the following
types of raster image files: JPEG, GIF, TIFF,
PCX, BMP, TGA (Targa), and BIL (SPOT
satellite), see p. 80
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Example – registration points
• If you don’t know
the projection,
you can get the
registration by a
best-fitting
procedure.
• In this case you
should provide
as many
registration
points as
possible.
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Sources
• Sources of registration points: GPS, other
maps, bench marks, geographic data,
recognizable points on ground (state
boundaries), other spatially enabled data
(road vector file)
– What are the considerations here?
• Scan or download image, or get aerial photo
– read into MapInfo as table, register, set up
as layer
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Overall intent is re-projection!
• During a raster re-projection process, the
software recalculates the pixel values of the
source image to make them display correctly
in the destination image.
• In this resampling process, the idea is to
restore every pixel value of the image based
on the pixels around it
• MapInfo has two methods for calculating the
pixel values of the destination image:
– Cubic Convolution
– Nearest Neighbor
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Example
USGS: (L) An elevation image classified from a satellite image
of Minnesota exists in a different scale and projection than the
lines on the digital file of the State and province boundaries.
(R) The elevation image has been re-projected to match the
projection and scale of the State and province boundaries.
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What about this?
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Iterate, especially now
• Get some experience, add/ remove control
points
• Compare the results from different choices
• Should start to give a feel for what are
reliable control points (and what are not)
– Recall the discussions on what a ‘point’ is and
how a line centroid is determined, etc.
• Friday!
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Digitizing
• Your (or with the help of the software)
determination of sets of points, lines, etc. on
a map in front of you
– Also called Heads-up digitizing (versus from a
digitizing tablet)
• To minimize image distortion, only digitize
from map images with known projections or
rectified aerial photographs
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Example
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MapInfo (others similar)
• Autotrace - Snap and Trace – Friday!
– S-key
– T-key (autotrace)
– N-key (autonode)
• Heads-up – point and click
– Practice on simple maps, or simple regions on
complex maps
• Streaming…
• Combining…
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Smoothing/ Unsmoothing
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Converting
• Sometimes (often) you want to convert what
you’ve capture into another GIS data type
• Polylines to regions - Regions to polylines
– E.g. join multiple polylines, split at nodes,
• Objects to polylines
– E.g. a parcel of land, contains different land-use
types
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• User Guide, pp. 164-165
Summary
• Four base topics for GIS (for Science)
– Layering
– Buffering
– Registering
– Digitizing
• For learning purposes remember:
– Demonstrate proficiency in using geospatial applications and tools
(commercial and open-source).
– Present verbally relational analysis and interpretation of a variety of spatial
data on maps.
– Demonstrate skill in applying database concepts to build and manipulate a
spatial database, SQL, spatial queries, and integration of graphic and tabular
data.
– Demonstrate intermediate knowledge of geospatial analysis methods and
their applications.
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Reading for this week
• MapInfo Tutorials (esp.)
• MapInfo User Guide (also see text book for
relevant chapters if available)
– Chapter 3 (Basics, esp. Working with Layers in
the Layer Control, p. 57) - layering
– Chapter 7 (Drawing and Editing Objects, esp.
Editing, p. 170) - digitizing
– Chapter 10 (Buffering and Working with Objects,
p. 268) - buffering
– Chapter 12 (Registering Raster Images, p.324) registering
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Friday Feb. 10th
• Lab session with Max (I am away)
• 12pm-1:30pm (attendance will be taken)
• Hands on
– Layers
– Buffering
– Registering
– Digitizing
• See web page for example data sources to
work with
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Next classes
• Geocoding (encoding) with streets/
addresses
• Simple interpolation
• Lab on Friday (17th)
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