u.s. coast & geodetic survey/noaa steps to creating a nautical chart

U.S. Coast & Geodetic Survey/NOAASteps to Creating a Nautical Chart

Historic types of surveys required – Astronomical Observations– Land survey (triangulation)– Tide Gauges– Shoreline Mapping– Hydrography

Theodolite instrument for precise astronomical observations to determine latitude, longitude, and azimuth

Astronomical observations were necessary to determine the position and orientation of selected points in a survey network.

View through Telescope at desired star

Star aligned with cross-hair

Astromical Observations performed at this site to determine initial latitude, longitude, and azimuth.

Survey Network

TRIANGULATION

• Baselines (BL) 1 & 2 are measured

•All angles of the triangles are measured

•Sides of all triangles are computed

• Computed length BL2 is compared to measured length of BL2

ME

AS

UR

ED

BA

SE

LIN

E 1

Angle measured

Angle measured

Angle measured

Angle measured

ME

AS

UR

ED

BA

SE

LIN

E 2

= survey mark

STEPS

TIDE

GAUGES

• Tide zone boundaries shown in red and orange

• Within each zone tidal height and time correctors are equal

•Tidal data required to correct depths and to determine horizontal position of shoreline

=Tide gauge

4. Plane table moved to another location and procedure repeated

5. Points of intersection define position of distant objects

6. Shoreline is drawn through intersecting points; also range & bearing determine positions

1. Plane table is set-up over known point

2. Paper map correctly oriented

3. Distant objects sighted upon and lines drawn from present position toward distant objects

PLANE TABLE MAPPING

6 7.2

9.1

6.5

6.1

5.1

7

8.9

7.87 8

7.9

9.397

5

6

9.285.5

7.1

Depths (soundings) are measured continuously along dotted lines. Sextant angle fixes are taken at locations indicated by green bars.

9.59.4

5.7

8

5.2

8.59.2

9.5

9.1

8.8

8.5

8.3

87.9

7.6

7.2

7.2

HYDROGRAPHY

• Two sextant angles at periodic points along the launch’s path determine position of the launch

= survey mark

GEODETIC SURVEYSHORELINE SURVEYTIDAL SURVEY

= tide gauge

HYDROGRAPHIC FIELD SHEET