GELE Photogrammetry & Cartography — Stereoscopy, DEM and OrthophotoConcept Map

Concept

Stereoscopy

Sub Concepts

• Stereo pair formation
• Forward overlap (60%)
• Parallax measurement
• 3D model visualization
• Convergent and normal case photography

Relationship To Central

Foundation for extracting 3D information from overlapping photographs

Concept

Base-Height Ratio (B/H)

Sub Concepts

• Air-base measurement
• Flying height determination
• Height measurement sensitivity
• Vertical exaggeration factor
• Optimal B/H values for different applications

Relationship To Central

Controls geometric strength and vertical exaggeration in stereo models

Concept

Digital Elevation Model (DEM)

Sub Concepts

• Grid-based elevation data
• Triangulated Irregular Network (TIN)
• Photogrammetric extraction
• LiDAR acquisition
• InSAR radar methods
• Bare-earth elevation values

Relationship To Central

Primary output for terrain representation and orthorectification

Concept

Relief Displacement

Sub Concepts

• Vertical displacement due to elevation
• Radial displacement from nadir
• Image scale variation
• Tilt distortion effects
• Measurement errors in raw photos

Relationship To Central

Image distortion caused by terrain variation that orthophotos eliminate

Concept

Orthophoto

Sub Concepts

• Orthophoto production process
• Resampling methodology
• Uniform scale at specified datum
• Planimetric accuracy
• Orthomosaics
• Seamless base maps

Relationship To Central

Final rectified product combining photographic detail with cartographic accuracy

Concept

Orthorectification

Sub Concepts

• DEM-based correction
• Interior orientation parameters
• Exterior orientation parameters
• Geometric transformation
• Pixel resampling techniques
• Ground control points integration

Relationship To Central

Critical process that uses DEM to produce orthophotos

Concept

Applications in Philippine Surveying

Sub Concepts

• Land use mapping compliance with RA 4374
• Contour generation for topographic surveys
• Volume calculation for earthwork (PD 1529)
• Cadastral mapping (CA 141)
• Viewshed analysis
• Drainage and watershed delineation
• PPCS/UTM coordinate integration

Relationship To Central

Practical use cases aligned with PRC standards and national mapping

To

Parallax Measurement

From

Stereoscopy

Strength

strong

Relationship

Stereoscopic viewing enables human perception and measurement of parallax shifts between stereo image pairs

To

3D Coordinates

From

Parallax Measurement

Strength

strong

Relationship

Parallax values, combined with collinearity equations and camera parameters, directly determine object heights and 3D positions

To

Parallax Measurement Accuracy

From

Base-Height Ratio (B/H)

Strength

strong

Relationship

B/H controls the magnitude of parallax values; optimal B/H (0.5–0.7) provides good height sensitivity without excessive vertical exaggeration

To

Vertical Exaggeration

From

Base-Height Ratio (B/H)

Strength

strong

Relationship

Larger B/H causes greater vertical exaggeration, making terrain features appear taller and steeper in the stereo model

To

Digital Elevation Model (DEM)

From

3D Coordinates

Strength

strong

Relationship

Parallax-derived 3D points from stereo image pairs form the raw data that is gridded or triangulated into a DEM

To

Orthorectification

From

DEM

Strength

strong

Relationship

DEM is the essential input for orthorectification; it provides elevation corrections to remove relief displacement from raw imagery

To

Raw Photo Distortion

From

Relief Displacement

Strength

strong

Relationship

Relief displacement causes raw photos to have non-uniform scale and planimetric errors that make direct measurement impossible

To

Orthophoto

From

Orthorectification

Strength

strong

Relationship

Orthorectification process, using the DEM and camera parameters, geometrically corrects the raw image to produce a uniform-scale, map-accurate orthophoto

To

Orthomosaic

From

Orthophoto

Strength

strong

Relationship

Multiple overlapping orthophotos are edge-matched, blended, and seamlessly tiled into an orthomosaic covering a larger area

To

Cartographic Measurement

From

Orthophoto

Strength

strong

Relationship

Unlike raw photos, orthophotos have uniform scale and are free of relief and tilt distortion, allowing direct measurement of distances and areas like a map

To

Geometric Transformation

From

Interior Orientation

Strength

strong

Relationship

Interior orientation parameters (focal length, principal point) define the camera's internal geometry essential for geometric transformation during orthorectification

To

Geometric Transformation

From

Exterior Orientation

Strength

strong

Relationship

Exterior orientation (camera position and rotation angles) combined with interior orientation enables transformation of image coordinates to ground coordinates

To

Exterior Orientation

From

Ground Control Points (GCP)

Strength

strong

Relationship

GCP coordinates measured via GPS or conventional survey provide known ground positions essential for solving exterior orientation equations

To

Contour Generation

From

DEM

Strength

moderate

Relationship

DEMs are interpolated or traced to produce contour lines at regular intervals for topographic map representation

To

Volume Calculation

From

DEM

Strength

moderate

Relationship

DEMs enable calculation of terrain volumes, cut/fill quantities, and earthwork estimates required for engineering projects under PD 1529

To

Viewshed Analysis

From

DEM

Strength

moderate

Relationship

DEM grid values determine line-of-sight analysis, visibility zones, and intervisibility between survey stations or infrastructure points

To

Watershed Delineation

From

DEM

Strength

moderate

Relationship

DEM slope and flow-direction algorithms delineate drainage basins, ridge lines, and water-flow paths for hydrological analysis

To

LiDAR DEM Production

From

Photogrammetric DEM Production

Strength

moderate

Relationship

Both methods produce elevation data; photogrammetry uses image correlation while LiDAR uses active ranging; often integrated for improved DEMs

To

Parallax Extraction

From

Image Matching

Strength

strong

Relationship

Automated image correlation algorithms identify homologous points between stereo images to measure parallax shifts and extract 3D coordinates

To

Orthophoto Quality

From

Pixel Resampling

Strength

moderate

Relationship

Resampling methods (nearest-neighbor, bilinear, bicubic) affect orthophoto visual quality, sharpness, and radiometric accuracy

To

Orthophoto Georeferencing

From

PRS92/WGS84 Datum

Strength

strong

Relationship

Orthophotos must be georeferenced to the Philippine Reference System (PRS92) and WGS84 datum for compliance with national surveying standards

To

Orthophoto Coordinates

From

PPCS/UTM Projection

Strength

strong

Relationship

Philippine Positioning and Cadastral Survey (PPCS) uses UTM zones 51–53N for projected coordinate representation of orthophotos

To

Orthophoto Application

From

Cadastral Mapping (CA 141)

Strength

strong

Relationship

Orthophotos serve as base maps for property boundary delineation and cadastral surveys under the Land Registration Act (CA 141)

To

Orthophoto Thematic Mapping

From

Land Use Classification (RA 4374)

Strength

moderate

Relationship

Orthophotos with orthomosaics provide the base imagery for land use and land cover (LULC) classification per RA 4374 requirements

To

DEM-Based Volume Computation

From

Earthwork Calculations (PD 1529)

Strength

strong

Relationship

DEM provides elevation data essential for computing cut/fill volumes and material quantities in infrastructure and construction projects

To

Orthophoto Accuracy

From

Quality Assurance

Strength

moderate

Relationship

Quality verification includes horizontal accuracy testing against GCPs and vertical RMS validation of DEM elevations before product delivery

To

Stereoscopy Success

From

Stereo Overlap

Strength

strong

Relationship

Approximately 60% forward overlap ensures sufficient image similarity for stereo fusion and effective 3D model formation

To

B/H Ratio Optimization

From

Convergent Photography

Strength

moderate

Relationship

Convergent camera angles can increase effective B/H without increasing physical air-base, improving height measurement strength

To

Orthophoto Application Selection

From

DSM vs DEM

Strength

moderate

Relationship

Choice between DSM (includes surface objects) and DEM (bare-earth) affects orthophoto suitability for different surveying and planning applications