Spatial data creation is the process of generating, organizing, and managing geographically referenced information in a Geographic Information System (GIS). It involves converting maps, satellite images, GPS observations, and field survey data into digital datasets that can be stored, analyzed, and visualized. The quality of GIS analysis depends largely on the accuracy of spatial data creation.
1. Creation of Shapefile and Geodatabase
A. Shapefile
A Shapefile is one of the most widely used vector data formats developed by Esri for storing geographic features.
Definition
A shapefile stores the geometry and attributes of geographic features such as points, lines, and polygons.
Components of a Shapefile
A shapefile consists of several files:
.shp – Stores geometry (shape)
.shx – Shape index
.dbf – Attribute table
.prj – Coordinate Reference System (CRS)
.sbn/.sbx – Spatial index (optional)
Geometry Types
Point – Wells, schools, hospitals
Line (Polyline) – Roads, rivers, pipelines
Polygon – Districts, lakes, forests
Creation of a Shapefile
Open GIS software (QGIS or ArcGIS)
Create New Layer
Select geometry type
Choose Coordinate Reference System (CRS)
Add attribute fields
Save the file
Start editing and digitize features
Advantages
Simple format
Highly compatible
Easy to exchange
Widely supported
Limitations
Only one geometry type per shapefile
Field names limited to 10 characters
Maximum size about 2 GB
B. Geodatabase
Definition
A Geodatabase (GDB) is a database designed specifically for storing and managing spatial and non-spatial data together.
It supports:
Multiple feature classes
Raster datasets
Topology
Relationship classes
Networks
Domains
Subtypes
Types
File Geodatabase (.gdb)
Personal Geodatabase (.mdb)
Enterprise Geodatabase
Advantages
Stores large datasets
Faster processing
Better data integrity
Supports topology
Supports multiple users
Difference between Shapefile and Geodatabase
| Shapefile | Geodatabase |
|---|---|
| Collection of files | Single database |
| One geometry type | Multiple feature classes |
| Limited field names | Long field names |
| No topology | Supports topology |
| 2 GB limit | Much larger storage |
2. Georeferencing
Definition
Georeferencing is the process of assigning real-world geographic coordinates to an image or scanned map.
Without georeferencing, an image has only rows and columns of pixels.
After georeferencing, every pixel represents an actual location on Earth.
Purpose
Align scanned maps
Overlay multiple datasets
Prepare historical maps
Integrate satellite images
Ground Control Points (GCPs)
A Ground Control Point is a known location identified both on the image and on the Earth's surface.
Examples
Road intersections
Bridges
Survey monuments
Building corners
Transformation Methods
Affine Transformation
Polynomial Transformation
Projective Transformation
Thin Plate Spline
Accuracy
Measured using
Root Mean Square Error (RMSE)
Lower RMSE indicates higher positional accuracy.
3. Image-to-Image Rectification
Definition
Image-to-image rectification aligns one image with another already georeferenced image.
Instead of using ground control points from field surveys, matching points are selected from the reference image.
Purpose
Register satellite images
Multi-temporal analysis
Change detection
Image mosaicking
Applications
Land use change analysis
Forest monitoring
Urban expansion studies
Disaster assessment
Difference
| Georeferencing | Image-to-Image Rectification |
|---|---|
| Uses map coordinates | Uses another image |
| Requires GCPs | Requires tie points |
| First image registration | Aligns additional images |
4. Spatial Data Integration (Digitization / Vectorisation)
Spatial Data Integration
Spatial data integration combines datasets from multiple sources into a unified GIS database.
Sources include
Satellite imagery
GPS surveys
Drone images
Paper maps
Census data
Digitization
Definition
Digitization is converting analogue maps into digital vector features.
It may be
Manual digitization
On-screen digitization
Steps
Scan map
Georeference
Create new layer
Trace features
Save edits
Vectorisation
Definition
Vectorisation converts raster images into vector data automatically or semi-automatically.
Raster → Vector
Example
Roads extracted from satellite imagery become line features.
Types
Manual Vectorisation
Automatic Vectorisation
Semi-automatic Vectorisation
5. Editing of Spatial and Non-Spatial Data
A. Spatial Data Editing
Spatial editing modifies the geometry of GIS features.
Examples
Move vertices
Add vertices
Delete vertices
Split polygons
Merge polygons
Reshape boundaries
B. Non-Spatial Data Editing
Non-spatial editing modifies the attribute table.
Examples
Population
Land use category
Road name
Soil type
Elevation
Attribute Editing Operations
Add field
Delete field
Update records
Join tables
Calculate fields
6. Data Editing – Removal of Errors
Spatial datasets often contain errors during digitization.
Common errors include
Overshoot
Undershoot
Sliver polygons
Duplicate features
Gaps
Overlaps
Importance
Improves accuracy
Prevents topology errors
Better spatial analysis
Better map quality
7. Overshoot and Undershoot
Overshoot
Definition
An overshoot occurs when a line extends beyond its intended intersection.
Example
A road continues past another road instead of ending exactly at the junction.
Problems
Incorrect network analysis
Connectivity errors
Correction
Trim the extra portion.
Undershoot
Definition
An undershoot occurs when a line stops short of an intersection.
Example
Two roads appear connected visually but actually have a gap.
Problems
Broken network
Routing failure
Correction
Extend the line or use snapping.
8. Snapping
Definition
Snapping is a GIS editing function that automatically aligns vertices, edges, or endpoints of features within a specified tolerance.
Purpose
Eliminate gaps
Improve connectivity
Maintain topology
Increase positional accuracy
Types
Vertex snapping
Edge snapping
End-point snapping
Example
When digitizing roads, the endpoint automatically snaps to an existing intersection.
9. Clipping
Definition
Clipping extracts only those features that lie inside a specified boundary (clip layer).
Think of clipping as using a cookie cutter.
Input
Input layer
Clip polygon
Output
Features inside the polygon only.
Applications
Extract district boundary
Watershed extraction
Protected area mapping
10. Intersection
Definition
The Intersection operation extracts only the common area shared by two or more layers.
Only overlapping portions remain.
Example
Intersect
Land use map
Soil map
Output
Land-use classes occurring on each soil type.
Applications
Suitability analysis
Overlay analysis
Environmental assessment
11. Union
Definition
The Union operation combines all spatial features from two polygon layers while preserving the geometry and attributes of both.
Unlike Intersection, Union keeps both overlapping and non-overlapping areas.
Example
Union of
Forest map
Administrative boundary
Output
All polygons with combined attributes.
Applications
Resource inventory
Land management
Spatial planning
Comparison of Overlay Operations
| Operation | Output |
|---|---|
| Clip | Features inside the boundary only |
| Intersection | Common overlapping area only |
| Union | Entire area of both layers with merged attributes |
Spatial Data: Data representing the location, shape, and relationships of geographic features.
Attribute Data (Non-Spatial Data): Descriptive information associated with spatial features (e.g., population, land use).
Vector Data: Geographic data represented by points, lines, and polygons.
Raster Data: Geographic data represented as a grid of pixels.
Coordinate Reference System (CRS): A system that defines how spatial data are referenced on the Earth's surface.
Ground Control Point (GCP): A location with known real-world coordinates used for georeferencing.
Topology: Rules that define spatial relationships such as connectivity, adjacency, and containment.
Digitization: Conversion of analogue maps into digital vector data.
Vectorisation: Conversion of raster images into vector features.
Georeferencing: Assigning geographic coordinates to an image or map.
Rectification: Correcting geometric distortions in an image to align it with a reference coordinate system or another image.
Snapping: Automatic alignment of vertices or edges during editing.
Overshoot: A line extending beyond its intended intersection.
Undershoot: A line failing to reach its intended intersection.
Clip: Extracts features within a specified boundary.
Intersection: Retains only the overlapping portions of input layers.
Union: Combines all features from polygon layers while preserving all spatial and attribute information.
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