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Scale, distance, area, etc. Expressed as a representative fraction 1:x or 1/x some agencies use equivalent distances as scale descriptors: e.g. 1

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Scale, distance, area, etc. Expressed as a representative fraction 1:x or 1/x some agencies use equivalent distances as scale descriptors: e.g. 1 =1 Mile (or ... – PowerPoint PPT presentation

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Title: Scale, distance, area, etc. Expressed as a representative fraction 1:x or 1/x some agencies use equivalent distances as scale descriptors: e.g. 1


1
Scale, distance, area, etc.Expressed as a
representative fraction 1x or 1/xsome agencies
use equivalent distances as scale descriptors
e.g. 11 Mile (or inch to the mile)
Small scale covers a large area Large scale
covers a small area objects appear
small objects appear large 1/1000 1/250,000
2
  • Map scale One inch on the map represents One
    mile on the Earth is represented on the map by
  • 11,000 83.42 feet 63.36 inches
  • 11,200 100 52.80
  • 12,000 186.24 31.68
  • 12,400 200 26.40
  • 12,500 208.56 25.30
  • 14,800 400 13.20
  • 15,000 417.12 12.67
  • 16,000 500 10.56
  • 19,000 750 7.04
  • 19,600 800 6.60
  • 110,000 834.24 6.34
  • 112,000 1000 5.28
  • 115,840 1320 4
  • 120,000 1668.48 3.17
  • 124,000 2000 2.64
  • 125,000 2085.60 2.53
  • 131,680 0.500 miles 2
  • 150,000 0.789 1.27

3
  • Map scale One cm on the map represents One km on
    the Earth is represented on the map
  • 11,000 10 meters 100 cm
  • 11,200 12 83.33
  • 112,000 120 8.33
  • 115,840 150 6.32
  • 120,000 200 5
  • 124,000 240 4.17
  • 125,000 250 4
  • 131,680 317 3.16
  • 150,000 500 2
  • 162,500 625 1.60
  • 163,360 634 1.58
  • 180,000 800 1.25
  • 1100,000 1000 1
  • 1125,000 1250 0.80
  • 1126,720 1267 0.79
  • 1250,000 2500 0.40
  • 1500,000 5000 0.20

4
Photo Scale
  • RF f/H
  • The focal length of the lens divided by the
    flying height of the platform above the ground.
  • Use the same units (units cancel as scale is
    unitless)
  • Dont forget to convert MSL altimiter readings to
    AGL before calculating scale.

5
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6
Photo distance/ground distance
  • RF pd/gd
  • Easy to remember...
  • Convert all distances to same units (units
    cancel)
  • Use base map to calculate ground distance.

7
pd/gd contd.
  • Be aware of map scale when calculating gd!
  • It is possible to calculate photo scale
  • RFpd/mdRF of map. (map distance photo scale)
  • I NEVER do it this way my arithmetic is too
    shaky and I feel a lot more comfortable seeing
    the distance.

8
Changes in scale across a single photo?
  • If the elevation of the target area changes, the
    scale changes
  • If the platform is subject to yaw-pitch or roll
    the scale will change across a photograph
  • Contract photography specify /- 3 of vertical
    and altimeter corrections every 20 minutes (at
    least)

9
Objects of a known size
  • If you have objects of a known size (or
    previously measured distances) present on a
    photograph, these can be used to calculate/verify
    photo scale.
  • Athletic fields, railroad gauges, automobiles,
    aircraft etc.

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14
Area Measurements
  • Best done from maps Planimetrically correct,
    known accuracy (national map accuracy standards.
    e.g. 1/24000 map objects will be within 20 of
    their true location)
  • No distortion due to topographic change
  • Air photos can be used if the topographic change
    is less than 100 meters (usual for most
    applications) OR calculate the scale for each
    elevation.

15
Grids
  • It is possible to calculate areas by overlaying a
    grid of known dimensions and simply counting the
    grid cells covered by the feature of interest.
  • How to handle corners?

16
Dot Grids
17
Using a dot grid.
You must know the photo scale Calculate the acres
(square meters, hectares) /square inch of the
photograph Use a dot grid that provides at least
1-2 acres/dot Drop the dot grid a couple of
times with different orientation and average to
get more accurate estimation of area
18
The polar planimeter
  • http//www.nas.com/kunkel/java/planimtr.htm
  • And http//persweb.wabash.edu/facstaff/footer/plan
    imeter/PolarLinear.htmHow20they20work

19
How Polar Planimeters are Used planimeters are
used by moving the tracer point around the
boundary of the region being measured.  As the
tracer point moves the wheel partially rolls and
partially slides on the paper, recording its
motion perpendicular to the tracer arm.  It turns
out that when the tracer point returns to the
point where it started, the net roll of the wheel
is proportional to the area of the region.  If a
scale attached to the wheel is calibrated
appropriately, the area can be read from the
scale. 
20
Transects quick and dirty estimations of land
cover
  • Divide photograph into equal strips
  • Count the approximate number of inches of each
    strip that are contained in each land cover
    class
  • For example
  • On a 9X9 photo, draw 9 equally spaced lines
    creating 10 strips (90 of transect) use a ruler
    to measure the woodland cover in a given strip.
  • Woodland covers approximately 30
  • 1/3 of the photo is forest when you know the
    photo footprint (a function of the scale) you
    can quickly estimate a given land cover type no
    equipment necessary.

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22
Digitizing the scanned photo is one of the most
accurate methods of area estimation Scale is
critical and the units displayed are (as yet)
unknown in their origin!
23
Measuring irregular line lengths
  • The digitizer distance function
  • Or the
  • Opisometer(Opisom"eter) n. Gr. backwards
    -meter. An instrument with a revolving wheel for
    measuring a curved line, as on a map also called
    a map wheel or road runner

24
Object count/Density
  • Closely related to texture.
  • If objects are regularly spaced, establish a
    number per unit area and calculate area.
  • e.g. cars in a parking lot
  • Standard density/area aids (forestry
    applications)
  • 5 acre ring, dot size grid, crown density grid
    (dependent on photo scale!!)

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27
Compass Bearings
  • Use map relationships to identify angular
    directions the angular relationships are well
    preserved in aerial photography.
  • If airfields are present every runway is marked
    with compass heading (magnetic heading)
  • Drop 1 zero the other end of the runway will be
    marked with the reciprocal bearing
  • e.g. 1 and 19

28
This end of the airstrip indicates a bearing of
140 degrees (SE) what is the reciprocal bearing
for the other end of the strip?
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