When one has to work with a paper map that doesn’t have UTM gridlines printed on it, at least three options are available:
• Visually estimate gridline positions using the grid tick marks.
• Physically overlay a scaled UTM grid transparency on the map.
• Physically draw UTM gridlines on the map.
Each of these options is discussed below.
3.2.1 Visual Estimation of Gridline Positions
Visually estimating (i.e., “eyeballing”) gridline positions using the corresponding pairs of grid tick marks on opposite margins of the map sheet is the least accurate of the methods considered here. It’s only suitable for obtaining very rough estimates, so this method generally isn’t recom- mended. However, this method is the least time-consuming, and with practice one can become more accurate.
3.2.2 Overlaying a UTM Grid Transparency
A UTM grid transparency is a sheet of transparent synthetic material (e.g., acetate film, polyethylene film), with sets of easting and northing gridlines printed or drawn on it. One overlays the transparency on the map so that the gridlines on the transparency align with the corresponding pairs of UTM grid tick marks on the map collar. Then one temporarily fastens the transparency to the map sheet so that the two remain fixed in relative position. A sepa- rate transparency is required for each map scale. Accurately plotting features on a map while using a transparency is impractical, but one can read existing map features and measure their UTM coordinates. Using a transparency has the advantage that it doesn’t require permanent modification of the map sheet.
Grid transparencies can be a variety of sizes. A full-size transparency covers the entire map image, whereas a small transparency might only be 45 square centimeters (7 square inches). For serious map work, where high accuracy is desired, a full-size transparency should be used; the large size allows one to accurately align the transparency grid with the map’s UTM grid tick marks.
Using a full-size grid transparency in the field can be awkward, for the following reasons:
• Large film sheets are awkward to store and transport. Much more so than paper maps, film sheets are damaged by folding, so transparencies should be stored flat or loosely rolled.
• Unsecured large film sheets are easily disturbed by wind gusts, which can easily dislodge or transport them.
• Using a large film sheet requires a large level work surface, which may not be available.
Thus, using full-size transparencies is recommended for office use only.
When using a transparency in the field, take steps to protect it from unnecessary exposure to abrasive debris and degrading UV radiation.
UTM grid transparencies can be constructed or purchased. Construction involves drawing or printing UTM gridlines on transparent film. Transparent film can be purchased in sheets and rolls of various widths, thicknesses, and compositions.
Constructing a transparency by drawing the gridlines directly on film requires drafting equip- ment, and is time-consuming and subject to error. Furthermore, constructing additional trans- parencies (e.g., to replace worn transparencies) requires repeating the drawing effort.
Constructing a transparency by printing gridlines on film involves several steps. First the film is acquired, measured, and cut to size. Then an electronic image of the gridline overlay is acquired. Finally, the gridline overlay image is printed onto the film.
An electronic image of the gridline overlay can be acquired in various ways, including the following:
• Download a grid image file from the worldwide web. For example, New Mexico Bureau of Geology and Mineral Resources [2007] makes some image files for small overlays available to the public in Portable Document Format (PDF).
• Draw the gridlines directly on a sheet of paper or film, scan the drawing, and store it as an image file.
• Use basic computer drafting/graphing software to draw the grid, and store the drawing as an image file. This is the preferred alternative, because it’s possible to draw the gridlines more accurately and consistently using computer software than by hand.
Additional, identical transparencies can be printed at will using the saved overlay image file without repeating the drawing effort. Once a gridline image file is created for one map scale (e.g., 1:24 000), it’s relatively easy to make a similar additional image file suitable for a different map scale (e.g., 1:100 000) by copying the original image file and then editing the copy.
3.2.3 Drawing UTM Gridlines on Maps
UTM gridlines can be drawn on a map in the drafting room prior to entering the field. Once the gridlines are drawn, no transparent overlay is required, nor is one present to interfere with plotting. Thus, using a map with pre-drawn gridlines is a convenient and reliable option for field work.
In some cases, permanently modifying a map sheet by drawing UTM gridlines on it might be considered undesirable, for the following reasons:
• The gridlines change the map’s overall appearance, possibly degrading its aesthetic appeal.
• The gridlines might make it difficult or impossible to read small graphic symbols, alphanu- meric labels or other details, thus degrading the map’s usefulness.
Drawing UTM gridlines on maps should be completed prior to entering the field, and under controlled conditions, so that the gridlines can be drawn neatly and accurately.
Select a work area that has good lighting and good ventilation, with a comfortable, adjustable chair and a large, clean work surface such as a professional drafting table. Wash your hands before you handle the map sheet and drafting equipment. Then fasten the map sheet securely to the drafting table.
If you have access to professional drafting equipment, use a good T-square as your straight- edge. Otherwise, use a straightedge with a thin cork backing so that (1) the straightedge doesn’t slip during use, and (2) the lower surface of the straightedge is slightly elevated off the map sur- face so the ink doesn’t wick under the straightedge during drawing. For drawing gridlines on USGS 7.5-minute quadrangle maps, the straightedge should be at least 60 cm (24 in.) long; for 15-minute quadrangles it should be at least 45 cm (18 in.) long. For larger maps, a longer straightedge is required. If the straightedge is too long, it will be awkward to use. A 60-cm (24-in.) straightedge is suitable for use with both 7.5-minute and 15-minute USGS quadrangle maps.
Use a technical pen with a fine point so the resulting UTM gridlines are thin and uniform. Excessively thick gridlines reduce a map’s usefulness by (1) needlessly concealing details on the map, and (2) reducing the precision of the gridline locations. Use waterproof ink so the gridlines won’t smear or wash out if the map is inadvertently exposed to moisture.
Prior to drawing any gridlines, fill the pen’s ink reservoir so that the ink supply won’t be exhausted while drawing a gridline. If the ink supply is exhausted midway through the drawing of a gridline, it may be difficult to draw the gridline neatly.
Before you begin drawing, practice drawing a few gridlines on an old map sheet or scratch paper. Try to determine (1) the optimum amount of downward pressure to apply to the pen, (2) the optimum speed to move the pen tip across the paper, and (3) the best position for holding the pen, for drawing gridlines neatly. For optimum results, practice using the same type of paper as the map sheet.
Visually locate all of the UTM grid tick marks on the map before you begin. It helps to systematically plan the order in which you draw the gridlines to minimize the time spent waiting for the ink to dry and to minimize contact with the finished parts of the map. The UTM gridlines
that run east-west across the width of the map should be drawn in order from most northerly (i.e., top of map sheet) to most southerly (i.e., bottom of map sheet). This will eliminate the need to lean over the gridlines after they’ve been drawn. Similarly, the UTM gridlines that run north-south across the height of the map should be drawn in order from right to left, or vice versa.
Allow each gridline to dry thoroughly before touching it with your hands or any drafting tools (e.g., straightedge), to prevent accidental smearing. After drawing the first set of gridlines (i.e., those that run east-west across the width of the map), allow the ink to dry thoroughly before drawing the second set (i.e., those that run north-south across the height of the map). While waiting for the ink to dry, check the pen’s ink reservoir and refill it if necessary.