#Fix Directive

While most survey vectors are established with compass and tape (CT) or RECT data lines, where FROM and TO stations are both specified, the SRV format also supports the defining of a special kind of vector via a directive line:

#Fix <name> <East> <North> <Up> [<variance override>]  [/<note>]  [<seg override>]

This establishes (or estimates) the location of a named station with respect to an implied zero reference. The three components are decimal numbers whose units are established by the Meters/Feet argument of a prior #Units directive.  (Or you can use the "M" or "F" suffix to indicate units.)  A #Units parameter, for example "order=NEU", can also specify the coordinate arrangement. The default is "ENU". If there are no elevations, omit the "U" and zero values will be assumed.

As a convenience, a slash-prefixed note is allowed on a #Fix data line, just prior to the optional segment override. The text following the slash is the same as what you would otherwise have to specify with a separate #Note directive.  (CAUTION: In the event your note contains the string "#S", you'll need to use a separate #Note directive.)

Also, #FIX directives are unlike ordinary two-station data lines in that a default flag assignment is applicable. For example:

#units        meters order=EN

#flag        /GPS Fixes (No elevations)

#fix                GPS9        620765                3461243        (R5,?)                /Bat Cave Entrance

#fix                GPS10        620550                3461133        (R5,?)                /Filled Sink #10

.....

#flag

The last #Flag directive removes the default flag assignment for fixed stations.  Alternatively, we could have replaced the default with a new one. Although a #Fix station can have many flags assigned to it in the usual way, only one default flag can be in effect when a #Fix directive is processed.  Note that the #Units directive also supports a "flag=<"flag name"> parameter, making it easy to set the default flag for entire project tree branches.

In this example we've also included variance assigments, "(R5,?)", to insure that the GPS positions won't be completely constrained when the survey is adjusted by least-squares. The "R5" specifies that Walls, when computing component variances, should assume an estimated Root-Mean-Squared (RMS) error in horizontal position of 5 meters. (The RMS error is often interpreted as the radius of a 63% confidence region.) The "?" specifies that no elevations were obtained.

The zero reference, referred to as "<REF>" in Walls dialogs and coordinate listings, is considered to be the same geographical location for all fixed stations in a compiled portion of a project.  Also, the East-North coordinates are assumed to be based on true north unless a correction, GRID=<true north minus grid north>, was specified in a prior #Units command or was established with a geographical reference.

UTM and Latitude/Longitude

When #FIXed points correspond to GPS position estimates (or locations scaled from maps), the horizontal coordinates can be longitude and latitude or, alternatively,  the easting and northing of a Universal Transverse Mercator (UTM) or Universal Polar Stereographic (UPS) projection. Such numbers alone, however, are not sufficient to uniquely define a position. During processing, Walls needs to know the correct geodetic datum and, if applicable, the UTM zone number. The way we currently provide that information is to use the Geographical Reference Page of the Properties Dialog to establish a geographical reference.  Normally you'll need to do this only once for the root folder of a project tree. Child items would then inherit this setting. Very large projects, however, might have separate georeference positions assigned to the different branches; and it's even possible that those references will have different geodetic datums and/or UTM zone numbers. (The latter situation could happen if the surveys straddle a zone boundary). Once a reference is supplied, the correct GRID correction (in this case the convergence angle) is supplied automatically as the initial default for each survey file.

To illustrate possible formats, here is a data file containing  some equivalent specifications for stations located at the same place:

; NOTE: The geographical reference assigned or inherited by this data file must have

; datum WGS84 and UTM zone 14 for the following fixes to be correctly interpreted.

#Units  meters  order=ENU

#FIX    A1   W97:43:52.5    N31:16:45         323f     /Entrance  ;dms with ft elevations

#FIX    A2   W97:43.875     N31:16.75         323f     ;dm

#FIX    A3   W97.73125      N31.2791667   323f     ;d

#FIX    A4   620775.38       3461050.67      98.45   ;UTM zone 14

#Units  feet  order=NE

#FIX    A5   3461050.67m  620775.38m    323      ;m suffix overrides ft units.

Note that latitudes and longitudes are specified with a letter prefix:  N or S for latitudes and E or W for longitudes.

Apart from supplying geographical fixes like those above, two other requirements must be met before UTM maps and coordinate listings can be created. First, before compilation,  make sure that the option UTM grid-relative is checked on the Geographical Reference property page. You must do this in addition to supplying a reference location. (One might want true-north coordinates with the geographical reference used only to compute declinations from dates.)  Second, before opening the Review dialog, make sure that Meters is selected as the Review Units on the General property page (unless you really want UTM-relative feet coordinates.)

As suggested above, Walls supports the automatic conversion of coordinates in one geodetic datum and/or UTM zone, to coordinates in a different datum and/or zone. This can happen when a child node of the project tree is assigned a reference different from that of the parent node being compiled. It's the reference assigned to the compiled node that determines the coordinate system of the generated data set.

#FIXed Points - General Properties

When #FIX directives are present they establish a network component's default grid origin at the zero reference.  So including at least one #FIX directive in your data is one way to have control over the origin of grid lines on maps.  (Another way is to set it explicitly via the Grid Intervals dialog.)  If a component has no #FIXed stations, then the default grid origin as well as the zero reference for coordinate listings is set automatically at the first station encountered in a project tree traversal.  There will be no <REF> station in this case, and of course your project won't be georeferenced.

By default, the location of a #FIXed station is not moved by the adjustment operation.  However, if a variance override appears in parentheses, it assigns the component error variances for the "hidden" vector connecting the station to the zero reference.  Thus a #FIXed station is not necessarily constrained or held fixed -- it can even be floated.  (See Variance Assignments.)  In this respect, #FIXed vectors are not unlike rectangular coordinate (RECT) vectors.

In earlier versions of Walls, multiple definitions of the same #FIXed station would likely abort compilation and produce a nonspecific error message. This would happen if any of the station's coordinates were assigned the default zero variance more than once. Now, when such a duplicate is encountered, a warning message is logged and the new definition is ignored. Note that duplicate #FIXed points with positive component variances are not inconsistent and will be averaged.

The zero reference will not appear on maps generated by Walls, nor will the hidden vectors connecting it to the #FIXed stations be scaled and drawn as connecting lines.  (Neither will they be considered in length computations.)  However, when different #FIXed stations are connected by surveyed traverses, the hidden vectors themselves become involved in loop systems and we need some way to represent them on the Map page's preview map.  Consequently, each #FIXed station has a small square drawn around it representing the hidden vector.  The same color scheme used for traverse highlighting is used for the square.  The squares will not appear in the printed and displayed map frames that are generated.

A #FIXed hidden vector will acquire the current segment attribute unless a #Segment directive appears on the same line to override it.  While the assigned segment's line style and color are irrelevant in this case, the segment can have a role in determining whether or not a #FIXed station is marked and/or labeled.