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GIS data encoding
1 GIS without data?
Can you imagine GIS without data?
GIS without data is like a car without fuel.
2 Data stream
Data stream includes data encoding and data editing processes.
Analogue data (paper form) must be made digital (computerized) first.
Data encoding and integration can be lengthy because of different types, formats, scales, and resolutions.
3 Data encoding
Data need to be encoded for GIS being used.
Data encoding is the process of getting data into the computer.
Many methods are available.
Differs by GIS packages.
Data standards help.
3.1 Data encoding methods
Data source | Analogue or digital source | Encoding method | Example |
---|---|---|---|
Tabular data | Analogue | Keycoding, text scanning | Address lists of hotel guests |
Map data | Analogue | Manual digitizing, automatic digitizing, scanning | Historical maps |
Map data | Digital | Digital file transfer | DTM, digital topographic data |
Field survey | Analogue | Keycoding | National population census data |
Field survey | Digital | Digital file transfer | Data from meteorological station recording equipment |
Aerial photos | Analogue | Manual digitizing, automatic digitizing, scanning | Ski slope locations |
Aerial photos | Digital | Digital file transfer | Ski slope locations |
GPS | Digital | Digital file transfer | Navigation systems |
Satellite imagery | Digital | Digital file transfer, image processing | Forest condition data |
3.2 Analogue data
Analogue data must be made digital.
Keyboard entry (keycoding) via keyboard
- Small data sets and attributes
- Human errors, typographical errors
Digitizing of paper maps using a digitizing table
- Reflects topology of features
- Takes patience and practice
- Positional errors
3.2.1 Digitizing analogue data
3.2.2 Scanning analogue data
Scanning (automatic digitizing) using a scanner to create a digital raster file of lines (automatic line follower)
- Faster and easier
- Expensive equipment
- Optical distortion
- Scanning of unwanted information
- Amount of editing to produce data for analysis can be huge
3.3 Electronic data transfer
Get data digitally if available.
Often reformatting (data conversion) is necessary (e.g., unzip, .e00 file, etc.).
Data collected digitally from GPS receivers, total stations, data loggers, and handheld devices
- May be transferred wirelessly
- May be in GIS format by default (Bonus!)
3.4 Obtaining data from other sources
What data are available?
What will the data cost?
How will the data be supplied?
How long will it take to get it?
What format will the data be in?
4 Data editing
Once in GIS, corrections and manipulations are made.
Check for errors
- Errors in source data
- Errors from the encoding process
- Errors propagated during transfer and conversion
Compare with source data
4.1 Common digitizing errors
4.2 Before and after data editing
4.3 Data transformation
Reprojection may be necessary to overlay data spatially.
Common coordinate system must be used.
Data may require transformation.
Scale is only as good as the worst used (smallest scale) – Other data is generalized to decrease unnecessary details and the file size.
4.4 Topological mismatch between data in different projections
4.5 Results of repeated line thinning
4.6 Edge matching
4.7 Editing database
Attributes of a database must be maintained and updated to be useful.
In a geodatabase, the existing length and area attributes become updated automatically, but other attributes will need to be updated manually.
- Can edit values one at a time in the attribute table
- Can use the Field Calculator to assign a value to a field
5 Geocoding
Geocoding is the process of converting addresses into geographic coordinates.
An address, a zip code, or a place name is used to convert from a non-coordinate system to geographic coordinates.
Address matching is the process of geocoding street addresses to street networks based on address ranges for each street segment.
5.1 Typical problems in geocoding
- Address misspelled
- Different formats (e.g., capitalization)
- Different abbreviations (e.g., periods or not)
Standards are needed.
6 Reverse geocoding
Reverse geocoding is the process of converting geographic coordinates into addresses.