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CREATION OF SPATIAL DATA


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

  1. Open GIS software (QGIS or ArcGIS)

  2. Create New Layer

  3. Select geometry type

  4. Choose Coordinate Reference System (CRS)

  5. Add attribute fields

  6. Save the file

  7. 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

  1. File Geodatabase (.gdb)

  2. Personal Geodatabase (.mdb)

  3. 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

  1. Scan map

  2. Georeference

  3. Create new layer

  4. Trace features

  5. 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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