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Lecture 6 Data entry

Lecture 6 Data entry. Getting the Map into the Computer Get data in finished form Analog-to-Digital maps Digitizing Data Entry Editing and validation

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Lecture 6

Data entry

Getting the Map into the Computer Get data in finished form Analog-to-Digital maps Digitizing Data Entry Editing and validation Scanning Georeferencing data Field and image data Attribute data

GIS maps are digital not analog Maps have a communications function but... A map has a storage function for spatial data Somehow, the visually “stored” data must

get digital Real and Virtual maps

Data Conversion

Traditionally most of the cost of a GIS project One time cost Depends on reuse Requires maintenance

Finding Existing Map Data

Map libraries Reference books State and local agencies Federal agencies Commercial data suppliers e.g. EOSAT,

ETAK

GIS data can be

Purchased Found from existing sources in digital form Captured from analog maps by GEOCODING

Turning analog (paper) maps to digital Digitizing (vector) Scanning (raster) Field Data Collection

GPS Survey

Digitizing Captures map data by tracing lines from a map by

hand Uses a cursor and an electronically-sensitive tablet Result is a string of points with (x, y) values

Heads-up digitizing example

The Digitizing Tablet\

Digitizer cursor transmits

a pulse from an electomagnetic

coil under the view lens.

Pulse is picked up by

nearest grid wires under

tablet surface.

Result is sent to

computer after

conversion to

x and y units.

map

Digitizing Stable base map, Medium? Fix to tablet Determine coordinate transformation Trace features Proof plot Edit Clean and build

Digitizing

Cursor data entry Secondary tablet (menu/template) Voice command entry Point select Stream mode

Selecting points to digitize

Some common digitizing errors

Slivers Duplicate lines Duplicate nodes Unended lines Gaps

Slivers

Sliver

Real river

Digitizer #1

Digitizer #2

Unsnapped node

Over shoots and under shoots

Scanning

Flat bed Drum DPI File size

Scanning example

This section of map was scanned, resulting in a file in TIF format that was bytes in size. This was a file of color intensities between 0 and 255 for red, green, and blue in each of three layers spaced on a grid 0.25 millimeter apart. How much data would be necessary to capture the features on your map as vectors? Would it be more or less than the grid (raster) file?

15 x 15 cm (3.6 x 3.6 km)

grid is 0.25 mm

ground equivalent is 6 m

600 x 600 pixels

one byte per color (0-255)

1.08 MB

Scanning

Places a map on a glass plate, and passes a light beam over it

Measures the reflected light intensity Result is a grid of pixels Image size and resolution are important Features can “drop out”

Image’s coordinates

X

Y0, 0

X

Y0, 0

Image’s coordinates and data’s coordinates

X

Y0, 0

117 W, 34 N

Reality

X

Y0, 0

-117 x, 34 y

Define control points, image to map

X

Y0, 0

-117 x, 34 y

Image moves

Attribute data

Table with rows and columns Attributes by records Entries are called values

Attribute data entry errors

Spelling Alesandro Allessandro Alessandro Allessandro

Locate/reduce spelling errors Enter the column twice and compare Use frequency statistics