GE 184

• Map scale


  The relation between the map distance and reality, i.e. ground distance

  
• Map scale


  The ratio between a distance on a map and the corresponding distance on the ground/terrain

  
• Map scale


  An expression of how much the area represented has been reduced on the map
• Maps take up less physical space than the area that they represent
• The reduction in size is reflected by the scale of the map, and is usually represented as a ratio or a graphic scale
• Scale is considered to be the single most critical mathematical feature of any map
• The Upper Glen Road, the first image, on the ground is displayed clearly on the map, the second image

  
• Verbal scale


  Expresses in words the relationship between a map distance and a ground distance

  
• Graphic scale/Bar scale


  Shows directly on the map the corresponding ground distance

  
• Bar scales is probably the most common kind of scale found on maps, perhaps because of their graphical nature makes them easily understood
• Another great thing about bar scales is that they remain correct if the map is reduced or enlarged photographically
• The main drawback of bar scales is that they are usually short compared to the map itself, and hence measuring longer distances is difficult
• Representative fraction (RF) or Ratio scale


  Shows the relationship between one of any unit on the map and one of the same units on the ground

  
• The denominator of an RF is called the scale factor
• Conversion of scales


  1. Example 1: If 1 centimetre on a map represents 10 metres on the ground, the RF is 1:1000

  2. Example 2: If the RF of a map is 1:120, 2 map units represent 240 ground units

  3. Example 3: If the RF on a map is 1:1200, a ground distance of 360 metres corresponds to 30 cm on the map

  
• Conversion of areas


  1. If the RF on a plan is 1:5, a square of 1 unit on the plan will represent a ground area of 5 x 5 units

  2. Therefore, Ground Area = Plan Area x (Scale Factor)2
• Categories of Maps


  • By Scale: Large Scale, Medium Scale, Small Scale
  • By Function: General Reference Maps, Thematic Maps
  • By Subject: Survey Plans
• Large Scale Maps


  • Cover less area, show more details, have the least generalization, have lots of textual information
• Small Scale Maps


  • Cover more area, show less detail, have more generalization, limited textual information
• In Ghana, the standard large scale map is 1:2500, medium scale is 1:50,000, and small scale is 1:1,000,000

  
• General Reference Maps


  Basic aim is to show the locations of all features on the earth surface like roads, coastlines, water bodies etc.

  
• Thematic Maps


  Display the spatial pattern of a single attribute or the relationship among several attributes
• Survey Plan


  Describes different types of plans depicting legal boundaries, prepared for official registration purposes or private use
• Survey Plans for Registration Purposes

  • Prove the identity of a land, delineate boundary locations, building locations, physical features and other items of spatial importance, an important legal document
• Surveying is the science and art of making all essential measurements to determine the relative position of points or physical and cultural details above, on, or beneath the surface of the Earth, and to depict them in a usable form, or to establish the position of points or details
• Surveying


  The measurement of dimensional relationships, such as horizontal distances, elevations, directions, and angles, on the earth's surface especially for use in locating property boundaries, construction layout, and mapmaking

  
• Surveying is an art in that only a surveyor who possesses a thorough understanding of the surveying techniques will be able to determine the most efficient methods required to obtain optimal results over a wide variety of surveying problems
• Surveying is scientific to the degree that rigorous mathematical techniques are used to analyze and adjust the field survey data
• Principles of Surveying


  • Work from whole to part
  • A point can be located by at least two measurements
• Work from whole to part

  The main idea is to localize the errors and prevent their accumulation
• Absolute vs Relative positioning

  Absolute positioning uses known control points, relative positioning uses measurements from other points
• The purpose of surveying is to measure altitudes/heights, angles, and distances on the land surface so that they can be accurately plotted on a map, and for setting out measurements on the ground for construction, engineering works and boundary delineation

  
• Two Main Types of Surveying

  • Plane Surveying
  • Geodetic Survey
• Plane Surveying


  Considers the Earth's surface to be a horizontal plane or flat surface for all X and Y dimensions, with height dimensions referenced to mean sea level
• Geodetic Survey


  Considers the Earth's surface to be spherical for X and Y dimensions, with height dimensions referenced to the mean spherical surface of the Earth (mean sea level)
• Classes of Land Surveys

  • Topographic Surveys
  • Cadastral Surveys
  • Hydrographic surveys
  • Photogrammetric Surveys
  • Engineering Surveys

  
• No measure is exact, every measurement contains errors, the true value of a measurement is never known, and the exact size of the error present in a measurement is always unknown
• Sources of Errors


  • Instrumental
  • Natural
  • Personal
• Classification of Errors


  • Gross Errors
  • Systematic Errors
  • Random Errors
• Gross Errors


  Also called mistakes or blunders, much larger than other errors, caused by carelessness or incompetence