several miles of the river, below and including the
fortifications. In this service a number of gunboats belonging to the
fleet and to the mortar flotilla accompanied the Sachem, partly to
afford protection, and partly to draw the enemy's attention from the
operations of the surveyors. Mr. Gerdes commenced work with his party on
the 13th of April, and continuing for five consecutive days, made a
reliable map of the river and its shores from the 'Jump' to and
including Forts Jackson and St. Philip, with their outworks and water
batteries; the hulks, supporting the chain across the river, and every
singular and distinguishable object along its banks. The survey was made
by triangulation carried forward simultaneously on both sides of the
river. Two coast survey signals were found, the 'Jump telegraph post,'
and 'Salt-work's chimney top,' of which the geodetic relations were
known, and the work was founded upon a base line connecting these two
points. Sub-assistant Oltmanns, and Mr. Bowie as aid, were detailed for
the west shore, Mr. Gerdes and acting assistant Harris taking the
eastern side, while sub-assistant Halter observed angles from permanent
stations. The angular measurements were made with all kinds of
instruments found suitable to the locality. Only a few of the stations
were on solid ground, nearly all the shore being overflowed. Frequently
the members of the party were compelled to mount their instruments on
the chimney tops of dilapidated houses. In other places boats were run
under overhanging trees on the shore, in which signal flags were
hoisted, and the angles measured below with sextants. It was very
satisfactory, however, that the last measurement determined (leading to
the flagstaff on St. Philip) agreed almost identically with the location
given by the coast survey several years ago. It seemed to be a regular
occupation of the garrison in the fort, to destroy, during the
night-time, the marks and signals which were left daily by the party;
and for this reason, Mr. Gerdes caused numbered posts to be set in the
river banks, and screened with grass and reeds so that they could not be
found by the enemy in the dark. From these marks, which were separately
determined, he was enabled to furnish to Captain Porter the distances
and bearings, from almost any point on the river to the forts, and by
the resulting data the commander selected the positions for his mortar
vessels.

On the 17th day of April the mort