Lecture 10: Mapwork Calculations PDF

LECTURE 10: MAPWORK CALCULATIONS Magnetic declination, Magnetic bearing, Cross sections, Vertical exaggeration, Intervisibility MAGNETIC DECLINATION  angle between true north (TN) and magnetic north (MN)  This angle is calculated when the map is drawn, but the position of magnetic north cha...

LECTURE 10: MAPWORK CALCULATIONS Magnetic declination, Magnetic bearing, Cross sections, Vertical exaggeration, Intervisibility MAGNETIC DECLINATION  angle between true north (TN) and magnetic north (MN)  This angle is calculated when the map is drawn, but the position of magnetic north changes, so the angle between true north and magnetic north (the magnetic declination) will also change  need to calculate what the magnetic declination is for the current year  magnetic declination for the year the map was drawn on the map – left-hand side or at the bottom of the map  In SA (and in the rest of Africa) the MN is always west of TN and moves westward MAGNETIC DECLINATION  change is determined annually and is expressed in minutes  Magnetic declination can change in TWO directions:  Westerly – The magnetic declination will INCREASE (for example: the change is 4’W, then we will add the 4’W to the MD 21°37’W because they are both moving in the same direction)  Easterly – The magnetic declination will DECREASE (for example: the change is 2’ East , then we subtract 2’E from MD 21°37’W)  years the mapmaker used to get the mean (average) magnetic declination DIRECTION OF CHANGE (W – angle YEAR THE DECLINATION increases, E – angle decreases WAS RECORDED TRUE refers to HOW MUCH the magnetic NORT declination changes by each year – MAGNETIC H change is in minutes NORTH  STEPS TO CALCULATE MAGNETIC DECLINATION  Step 1: Work out the difference in years between the current year and year given on the map. Your answer must be in years. (Use the year that is printed straight after the words ‘true north’. You can ignore the month that is shown.)  Step 2: Multiply the number of years with the mean annual change (this is given on the map) to get the change since the declination was recorded.  Step 3: If the mean annual change is eastwards, then you have to subtract the change from the magnetic declination given. If the mean annual change is westwards, then you must add it to the given magnetic declination.  Step 4: Your answer is the magnetic declination for the current year. Magnetic declination is always west of true north. EXAMPLES EXAMPLES EXAMPLES MAGNETIC BEARING  the angle measured clockwise from magnetic north. Here the magnetic north line is taken as 0º whereas in true bearing, true north is taken as 0°  Formula: MB = True Bearing + Magnetic Declination EXAMPLES If the MD given on the map is recorded as a decimal, for example 23°,5 W, you must multiply the number after the comma by 6 to convert it to minutes. For example: 5 × 6 = 30’. So, the MD is now 23°30’ W. The decimal comma has been removed and you have a MD in degrees and minutes. EXAMPLES 78o 27‘   In mapwork, we draw a cross section (view from the side) of an area or landform to better CROSS SECTION understand it looks like  cross section is when we ‘cut’ through a landform, to see what it looks like from the side   Draw a straight line joining the two points in question HOW TO  Put a piece of paper with a straight edge along this line CONSTRUCT A  On this strip of paper mark off each contour line you cross and indicate its altitude CROSS SECTION  The position of any major features such as rivers, national roads, etc. should be noted HOW TO CONTSTRUCT A CROSS SECTION  Get graph paper and draw a horizontal and vertical axis on it  The height of the vertical axis depends on the contour readings on the strip of paper  On the vertical axis put a scale of 1cm = 20m and 1:50 000 on the horizontal scale  Make your bottom line one contour interval lower that the lowest height on your strip of paper. On the above graph the lowest height is 1 010m, therefore your bottom point on your vertical scale is 900m  Place your strip of paper along the bottom line and move it up keeping your starting point on the vertical axis. When the first height on your piece of paper corresponds with the same height on the vertical axis, mark its position with a dot on the graph  Join these dots to complete your cross section VERTICAL EXAGGERATION  A cross section is drawn on a graph – vertical axis to show the height and horizontal axis to show the distance  If the vertical and horizontal scales are the same, it is not easy to see the differences in slope  To overcome this problem, we exaggerate (make it more obvious or clear) the profile vertically by using a different vertical scale from the horizontal scale  the cross-section has been exaggerated – need to calculate how many times it has been made steeper or exaggerated (vertical exaggeration) VERTICAL EXAGGERATION EXAMPLES  used to determine whether one place INTERVISIBILIT is visible from another place – can Y you see one place from another place   Step 1: Draw a line joining the points between the two places METHOD TO  Step 2: Look to see if the line you have DETERMINE drawn cuts through any part of the cross section. If it does cut through, then there is INTERVISIBILITYno intervisibility between the two points. If it does not cut through, then there is intervisibility between the two points