The Graphic Overlay Method

Measuring Map Features
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There are a myriad of characteristic land and water patterns that must be identified.  Aerial photography is perhaps the least expensive and most practical method of effective land pattern identification currently available.  According to D. Way (1973), typical air photo interpretation characteristic pattern elements are:   
  • Topographic patterns are identified in terms of the degree of disection and continuity.
  • Drainage patterns are evaluated in terms of their type or texture.
  • Surface features are identified by their photographic tone and/or color.
  • Erosion features such as gullies are identified.
  • Vegetation and landuse features are identified.
  • Other miscellaneous features such as rock fractures, rock outcrops,and geobotanical indicators are identified.
What are the quantities of each map feature?  To be valuable to GIS applications each map attribute, map feature, or map component (deemed necessary) must be quantified in a unit of measure appropriate to that feature. 
Feature Categories, Measurements, and Conversions Index:
Use the Index (above) or scroll to view the listings. 

Map features are measured in individually appropriate units that best represent the land (or water) activity or use.  The typical units of measure are frequency, size, color, height, weight, uniqueness, and capacity.  Here are a few examples (of course), there are numerous features and forms of measurements not listed here. 

 Return to Categories, Measurements, and Conversions Index: 

  • Driest monthly precipitation,  inches/month 
  • Fog Visibility,  distance in feet 
  • Frost Thickness, thickness in inches 
  • Ground radiation, 
  • Soil temperature,  degrees fahrenheit
  • Maximum 24-hr. precipitation,  inches/day 
  • Maximum 24-hr. snowfall,  inches/day 
  • Mean annual precipitation, inches/year
  • Mean annual snowfall,  inches/year
  • Mean annual temperature,  degrees fahrenheit 
  • Mean monthly temperature,  degrees fahrenheit 
  • Mean relative humidity,  percent air saturation 
  • Number of days of precipitation, days (w and w/o) 
  • Potential evapotranspiration,  inches (loss) precipitation.
  • Solar radiation,  air temperature in degrees fahrenheit 
  • Storm systems,  direction and speed of travel 
  • Surface air drainage,  temperature (F.) and flow rate (cfs) 
  • Temperature (air),  degrees fahrenheit 
  • Wettest monthly precipitation,  inches/month 
  • Wind,  direction (N,S,E,W), and speed (mph)
Return to Categories, Measurements, and Conversions Index: 

  • Building materials Cubic yards (sand and gravel) 
  • Depth to bedrock Depth from ground surface in feet 
  • Economic minerals Depth (feet), overburden (tons), 
  • Land form types Most rare, least rare (Uniqueness) 
  • Landslide potential Prob. of occurrence (high/low) 
  • Rock outcrops and ridges 
  • Percent slope, and visual quality 
  • Rock types % of Sandstone, Limestone, Shale, 
  • Stability of bedrock Fracture potential 
  • Structural characteristics Hardness, weathering properties 
  • Unique land formations Most unique, least unique 
  • Unstable land formations Most stable, least stable 
  • Volcanic activity Probability of occurrence (high/low) 
  • Weathering properties 
  • Erosion potential, and severity 
Return to Categories, Measurements, and Conversions Index: 

  • Bearing capacity Pounds per square foot 
  • Erodability Most susceptible to least susceptible 
  • Instability High risk to low risk 
  • Internal drainage Volume (cfs), length (ft.) 
  • Permeability Very permeable to not permeable 
  • Productivity Highly productive to not productive 
  • Soil erosion Tons/year 
  • Soil type Classification by unique properties 
  • Soil depth By A, B, C-Horizon, strata in feet 
  • Soil texture V. coarse to v. fine particle size 
  • Stoniness Size and frequency per square foot 
  • Wetness hazard High possibility to no possibility.
Return to Categories, Measurements, and Conversions Index: 

  • Contour interval Feet 
  • Elevation Height (in ft.) above mean sea level 
  • Length of contour lines Miles 
  • Mean basin elevation Feet 
  • Orientation Aspect: North, South, East, West 
  • Relief variation Variation of vert./horizontal ratio 
  • Slope rim Location, aspect, and length in mi. 
  • Slopes Ratio of height/length in feet 
Return to Categories, Measurements, and Conversions Index: 

  • Aquifer recharge Thousands of gallons per second 
  • Bayous Visual quality, wildlife quality... 
  • Biological productivity Highly productive to not productive 
  • Drainage density Fine, medium, and coarse 
  • Drainage patterns Water energy potential in ft. lbs. 
  • Edge stability 
  • Erosion potential 
  • Flood potential Frequency of occurrence, water depth 
  • Gage elevation Feet 
  • Ground water depth Feet from surface to potable water 
  • Ground water quality Clarity (vis. depth), turbidity 
  • Ground water recharge Potential for recharge 
  • Ground water contamination Potential for occurrence 
  • Lakes Size in acres, depth in feet, 
  • Length of streams Miles 
  • Length of tributaries Miles 
  • Main stream length Miles 
  • Mean annual flood Cfs/square mile 
  • Rivers Length/depth in feet 
  • Shoreline use (activities) People per day 
  • Shoreline quality Absence or presence of trash 
  • Springs Volume in gallons per minute 
  • Streams Flow rate in feet per second 
  • Stream slope Vertical change in feet/mile 
  • Water clarity Dissolved solids Parts per milligram Suspended solids Size in mm.
  • Water quality (surface) Visual, smell, clarity, taste, sediment load, etc. 
  • Water temperature Degrees Fahrenheit 
  • Waterfalls Height and width in feet 
  • Watersheds Square miles 
  • Watershed slope Feet per mile 
  • Wetlands Visual quality, and wildlife habitat potential.
Return to Categories, Measurements, and Conversions Index: 

  • Agricultural Acres, productivity (active/abandoned) 
  • Composite land cover Acres / landcover category 
  • Edge communities Number and variety of species, uniqueness 
  • Fields Acres, wetness hazard, productivity 
  • Forests Acres, productivity 
  • Natural associations Uniqueness of species 
  • Overstory type Species type, height, density 
  • Understory type Shade tolerance, diversity 
  • Pastures Animal grazing months 
  • Riparian communities Uniqueness of species, acres/miles 
  • Sources of bldg. materials Quantity in board feet 
  • Sources of heating fuel Volume in chords 
  • Specimen areas Most to least vulnerable 
  • Vegetation type Species names 
  • Vegetation quality Age class, uniqueness, visual quality.
Return to Categories, Measurements, and Conversions Index: 

  • Major ecotones Indicator species, uniqueness 
  • Primary habitats Acres of forage, species variety 
  • Unique areas Most rare to least rare 
  • Wilderness areas Acre size, species diversity 
  • Wildlife types Species variety 
  • Wildlife habitats Contiguous acres, pathways and travelway linkages 
Return to Categories, Measurements, and Conversions Index: 
Typical Unit Conversions 

All units of measure can be easily converted to more practical or more familiar units.  GIS is not dependent on any specific units of measure, because "units" are easily converted or interchanged one unit to another.  

To convert: [Acres] to [Hectares] 
Multiply [Acres] by [.4047] 

  • Acres to Hectares .4047 
  • Centimeters to Feet .0328 
  • Cubic feet to Cubic meters .0283 
  • Gallons to Liters 3.7853 
  • Feet/Second to Meters/Second 8.4667 
  • Hectares to Acres 2.4710 
  • Feet to Meters .3048 
  • Meters to Feet 3.2808 
  • Miles/Hour to Kilometers/Hour 1.6093 
  • Miles to Kilometers 1.6093 
  • Sq. Miles to Sq. Kilometers 2.5899 
Return to Categories, Measurements, and Conversions Index: 

See how maps are coded. 

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All contents copyright (C) 1993-1999,  D. Fehler  All rights reserved.