The independent coordinates of horizontal control points are required in the state plane coordinate system for preparation of topographic map. So, data from field surveying are used to calculate and adjusted in the office, correct within specified limits, and made available for direct plotting by the coordinate method. This plotting can be performed with high precision, since all measurements are linear distances measured from orthogonal axes.

In order to plot by the coordinate method, a series of grid lines are drawn on the base sheet for the map using a graticule (Figure 32.1). These grid lines are sets of X and Y axes of orthogonal coordinate systems. In the state plane coordinate system (and in most other coordinate systems), the Y grid axis is aligned toward North. These grid lines are spaced at some regular, uniform interval say, 50, 100, 200, 500 or 1000 m suitable for the scale of the map being compiled. When drawing the grid lines, extreme care must be exercised to ensure that the lines are straight and of uniform weight and that the X grid lines are perpendicular to the Y grid lines. Each X and Y grid line is labeled at the edges of the map sheet with its coordinate value. Grid lines should be drawn as quickly as possible on stable-base material under uniform temperature conditions so that all measurements are consistent. The positions of the control points are then plotted by laying off the differences between the coordinates for the point and the coordinates of a pair of intersecting grid lines close to the control point (Figure 32.2).

The grid lines are retained on the finished drawing or map or at the very least, the grid tick intersections are left. These grid lines or grid intersection points are invaluable for scaling the coordinates of points on the map. They can also be useful to evaluate dimensional changes in the map-base material and allow more realistic scaling of distances and locations from a given map sheet.

The advantages of plotting horizontal control by rectangular coordinates are as follows:

(1) adjusted coordinates are used to plot the locations so it is known that the data are correct to within the closure of the control survey network;

(2) each point is plotted independently so that there is no accumulation of error;

(3) high precision and uniform accuracy can be maintained on large map sheets and when multiple map sheets are required; and

(4) the method is very adaptable to automatic plotting routines and is compatible with modern data-storage procedure.

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