3-6  Transverse Mercator Projection a.  General.  The Transverse Mercator Projection is a cylindrical conformal projection.  It is based on a modified Mercator Projection in that the cylinder is rotated (transversed) 90° so that the rotational axis of the ellipsoid is perpendicular to the axis of the cylinder.  Generally, the Transverse Mercator Projection is considered as a cylinder which is secant to an ellipsoid as described in paragraph 3-6c below. See Figure 3-4.  Only a six degree wide portion of the ellipsoid is projected onto the cylinder.  The centerline of the projected area is called the Central Meridian; the ellipsoid is then rotated six degrees inside the cylinder and another six degree portion is projected. Figure 3-4  Transverse Mercator Projection b.  Area Distortion.  When the Transverse Mercator Projection is used to project a portion of the ellipsoid onto the cylinder, the equator and the central meridian will appear as mutually perpendicular straight lines. See Figure 3-5.  A hemisphere will be distorted towards its outer edges.  The shaded areas of Figure 3-5 show the varying distortion of two equivalent geographic areas on the same projection.  Note that both areas encompass a region which is 20° by 20° and are both bounded by 20° and 40° N latitude; therefore, on the ellipsoid they are the same size.  On the projected surface; however, the area bounded by 60° and 80° longitudes is much larger than the area bounded by 0° and 20° longitudes.  To decrease the amount of distortion, the ellipsoid is divided into 60 six-degree wide projection zones, each having a meridian of longitude as its Central Meridian.  Each zone is projected between 84° 30' N latitude and 80° 30' S latitude.  See Figure 3-6.    c.  Secancy of the Ellipsoid.  The cylinder used as the projection surface for the Transverse Mercator Projection is generally considered to be secant to the Figur e 3-5 Disto rtion withi n the Tran svers e        Merc ator Proje ction ellips oid as show n in Figur e 3-4. This mean s that the cylin der inter sects the ellip soid in two place s creat ing lines of seca ncy whic h are paral lel to the centr al meri dian of the proje ctio n. DRAFT 3-4 Figure 3-6  Secancy in a 6° Zone 80° 80° 60° 40° 20° 0° 20° 40° 60° 40° EQUATOR 20° 6° 180,000 METERS 180,000 METERS