Standard Parallel:
In a projection there are certain constant properties are there, one of which is the Standard parallel,
defined as the line of latitude in a conic or cylindrical projection in normal aspect where the
projection surface touches the globe. A tangent conic or cylindrical projection has one standard
parallel, while a secant conic or cylindrical projection has two. At the standard parallel, the projection
shows no distortion.
Thus, Standard Parallels are lines at which there is no distortion in the map projection. Standard parallels
are used in both conical and cylindrical projections. They theoretically touch the earth's surface.
Standard parallels are parallel lines that are drawn on a reference globe that will maintain a scale factor
of 1.0, so a standard parallel is a line of latitude that has true scale. Not all projections have standard
parallels, but many common ones do. Conic projections often have two. In a few projections, like the
Sinusoidal and the Polyconic, every line of latitude has true scale and is therefore a standard parallel.
Standard parallel in conical projection plane
Selection of Standard Parallel:
As from the definition it is clear that the Standard Parallel can any line of latitude depending upon where
the projection plane is touching the globe. Thus, for each separate projection type standard parallel is
selected differently.
• Conical projection case:
A conical projection is one, which is drawn by projecting the image of the graticule of a
globe on a developable cone, which touches the globe along a parallel of latitude called the
standard parallel. As the cone touches the globe located along AB, the position of this
parallel on the globe coinciding with that on the cone is taken as the standard parallel. The
length of other parallels on either side of this parallel are distorted.
The case given above is for tangent conical projection or when the projection plane touches the
globe only on one parallel line or just two sides on two points. Then the line is selected as the
standard parallel, where the parallels above or below it is distorted in the sense as scale factor
changes. For selection of any parallel it is given that,
Example:
Construct a conical projection with one standard parallel for an area bounded by 10º N
to 70ºN Latitude and 10º E to 130º E Longitudes when the scale is 1:250,000,000 and
latitudinal and longitudinal interval is 10º.
Calculation:
Radius of reduced earth R = 640,000,000 = 2.56 cm 250,000,000
Standard parallel is 40º N (10, 20, 30, 40, 50, 60, 70)
Central meridian is 70º E (10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130)
Construction:
(i) a circle or a quadrant of 2.56 cm radius marked with angles COE is drawn as 10o
interval and BOE and AOD as 40o standard parallel.
(ii) A tangent is extended from B to P and similarly from A to P, so that AP and BP are
the two sides of the cone touching the globe and forming Standard Parallel at 40° N.
(iii) The arc distance CE represents the interval between parallels. A semi-circle is drawn
by taking this arc distance.
(iv) X-Y is the perpendicular drawn from OP to OB.
(v) A separate line N-S is taken on which BP distance is drawn representing standard
parallel. The line NS becomes the central meridian.
(vi) Other parallels are drawn by taking arc distance CE on the central meridian. (
(vii) The distance XY is marked on the standard parallel at 40° for drawing other meridians.
(viii) Straight lines are drawn by joining them with the pole.
The above shows the selection of SP based on single point or tangent, there is another case
where the conical plane touches the globe on two points or two lines of latitude, that case is
known as the secant conical projection where two standard parallels are present.
In case of secant, the distortion is minimum between the two selected SP, outside of them the
distortion increases.
• Cylindrical projection case:
In case of normal or tangent cylindrical projection, such as cylindrical equal area projection,
the cylinder is vertically surrounding the globe and thus touching the globe along a single line
of latitude, which is for the said projection is taken to be the equator. Thus, when unfolded, the
scale factor remains true or zero distortion occurs along the line and all other meridians are
equally spaced upon the SP.
Here , the first figure on the left shows the cylindrical projection with one standard parallel,
which when unfolded will look like –
On the other hand, in case of secant case there will be two SP with minimum distortion between
them, which when unfolded will be look like-
