What is Cartographic Scale
The ratio between the distance on the photo and the corresponding distance on the ground
- expressed as a representative factor (RF)
written as a ratio X/Y
x= numerator-photo
y=denominator-ground
3 Scale determination methods
- focal length & altitude
- photo-map distance
- ground distance
What is a large vs small cartographic scale
- The large scale map covers smaller area but generally more detail (denominator is smaller)
- The small-scale map covers larger area with less detail (denominator is higher)
- map scale: the proportion of a distance on a map to the corresponding distance on the ground.
Small vs large geographic scale
Observation scale: spatial extent of a study area
- a large geographic scale involves a large spatial area
- small geographic scale study only contains a small spatial area
What is Focal length?
distance from the middle of the camera lens to the focal plane (film)
What happens to image distortion as focal length increases?
As focal length increases image distortion decreases
What is Altitude
linear distance from the center of lens to the ground
How to calculate scale from focal length & altitude
- vertical photograph scale is a function of the focal length of the camera (F) and the height above the terrain (H)
RF= F (focal length)/H (altitude).
= average or nominal scale of the photograph
How to determine scale from photo-map distance?
RF= 1/ (md)(MS)/PD
Md= map distance btw 2 points
MS= map-scale representative fraction
PD+ photo distance btw the same 2 points
presented as a fraction1/- OR ratio 1: -
Scale determination from photo-ground distance
RF= 1/ (GD/PD)
GD= ground distance between 2 points
PD= photographic distance between 2 points
Area Determination
- Analog Air Photo = scanner
- Digital Air Photo = digital image processing system
- Image rectification (geometric) = on screen digitalization = perimeter & area
Mission Planning 8 Considerations
- Study Area
- Required weather & timing
- Film-filter combination
- film tilting
- flight lines
- Overlap/side lap
- airplane altitude
- flight-plan map
Study Area (planning)
- delineation of area to be photographed
- What is the study area- flight altiude
- digital imagery more flexible
Required weather (planning)
Wind: plane drifts off course
Clouds: less than 10% cloud cover (shadows)
Season: how much sun, what features of nature must be identified?
- topographic mapping, soils: cant have snow
- vegetation: mid spring to summer= leaves
- submersed features: water turbidity, marshes, flooding
Required time of day (planning missiokn)
sun elevation angle= f
- depends on latitude, season, time of day
- shadow is NOT wanted: 30-50 degrees
- shadow is wanted: <30 degrees sun elevation angle
sunspots: occur at high sun elevation angles
- straight line from sun passes through camera lens and intersects ground inside camera)
Film=filter Combination (planning)
B & W: cheapest
Color: natural= regular OR color infrared (false color)= more expensive
Filter: subtraction of light (filters out certain colors)
- light is filtered to obtain a narrow amount
- Film filter combination: removes atmospheric haze
Flight Lines (planning)
- parallel to the longest axis of study area
- extra flight line added to each side of study area
Overlap: 60% along flight line (stereoscopic)
Side lap: 20-30%between adjacent flightlines
