LAB 5: ISOLINE MAPS AND CROSS-SECTIONS
GRAPHING DATA WITH LOGARITHMIC SCALE
OBJECTIVES:
-To construct isoline maps, cross-sections and calculate gradients using climatological data.
-To introduce and use latitude and longitude.
-To graph data using a logarithmic scale.
As we have seen in Lab 3, one common problem in geography is the need to portray data that have been collected to show spatial variability. We have already seen how to draw isolines as contours (lines of equal elevation that were used in Lab 3). The data you will use in this lab are annual precipitation and topography for the island of Maui, HI.
I. CONSTRUCTION OF ISOLINES BY-EYE:
1. The point data must first be located on a map. In this example the precipitation data have already been plotted for you.
2. The isolines are then added to the map. In order to do this a series of decisions must be made. The first of which relates to the choice of interval at which the isolines are to be drawn. This may be decided by convention e.g. isobars (lines of equal pressure which you will be constructing on weather maps later this semester) are drawn at 4 mb intervals. Alternatively, judgment is used to select an interval that allows trends to be shown clearly but prevent the map from becoming too cluttered.
3. Next the isolines must be drawn on or between the known data points. Locating the exact positions of the isolines is an example of the process of interpolation (already referred to in Lab 1). Spacing can be determined precisely with the proportional distance method we have used in Lab 3. The by eye method is faster. For example, if an isohyets spacing of 50 mm is being used, the 50 mm isohyets would pass mid way between two points with precipitation 25 and 75 mm, yet much closer to a point 32.5 mm than one 87.5 mm. The positioning between these latter two points can be determined relatively precisely. If a second reference line were drawn between the 32.5 mm and 87.5 mm data points, the isohyets would lie 1/4 of the distance along the line from the 32.5 mm point and 3/4 of the distance from the 87.5 mm point. See diagram below.
NOTE: When constructing isoline maps make sure all isolines are clearly labeled.
II. LATITUDE AND LONGITUDE
The spinning of the Earth on its axis provides two natural reference points, the poles, upon which to base the geographic grid, a set of intersecting lines inscribed on the globe, for the purpose of fixing the location of surface features. The grid consists of a set of north-south lines, lines of longitude (meridians), and a set of east-west lines running parallel to the equator, lines of latitude (parallels).
Distances are measured in fractions of a circle (degrees). Remember that all circles can be divided in 360°. Thus both latitude and longitude are measured as angles, with units of degrees (°), minutes (') and seconds ("). There are 60 minutes in a degree, and 60 seconds in a minute.
The reference location for latitude is the equator, a circle defined by points halfway between the geographic north and south poles. Latitude ranges from 0° at the equator to 90°N at the North Pole and 90°S at the South Pole.
Viewing the earth from space located over the Equator
Longitude ranges 0-180°W, and 0°-180°E (eastern and western hemispheres). The reference line 0° (prime meridian) runs through Greenwich, U.K. All longitudinal positions are measured from this reference. Longitude increases both east and west from 0° at the prime meridian to 180°E and W (the same place), which lie in the middle of the Pacific. Because the meridians converge at the poles, the ground distance represented by a degree of longitude varies.
Viewing the earth from space located over the North Pole
