## DESCRIPTION

v.to.db loads vector map features or metrics into a database table, or prints them (or the SQL queries used to obtain them) in a form of a human-readable report. For uploaded/printed category values '-1' is used for 'no category' and 'null'/'-' if category cannot be found or multiple categories were found. For line azimuths '-1' is used for closed lines (start equals end).

## NOTES

Measures of lengths and areas are always reported in meters, unless the unit parameter is set. The units miles, feet, meters and kilometers are square for option=area.

Feet and acre units are always reported in their common versions (i.e. the International Foot, exactly 5280 feet in a mile), even when the location's standard map unit is the US Survey foot.

When calculating perimeters in Latitude-Longitude locations, the geodesic distance between the vertices is used.

When using option=coor on a vector area map, only coordinates of centroids with unique category will be reported.

When using option=bbox on a vector area map with more than one feature per category value, the results corresponds to the bounding box of all features of same category taken together.

Line azimuth is calculated as angle from the North direction to the line endnode direction at the line statnode. By default it's reported in decimal degrees (0-360, CW) but it also may be repored in radians with unit=radians. Azimuth value -1 is used to report closed line with it's startnode and endnode being in same place. Azimuth values make sense only if every vector line has only one entry in database (unique CAT value).

If the module is apparently slow and the map attributes are stored in an external DBMS such as PostgreSQL, it is highly recommended to create an index on the key (category) column.

Uploading the vector map attributes to a database requires a table attached to a given input vector layer. The print only (-p) mode doesn't require a table. Use db.execute or v.db.addtable to create a table if needed.

Updating the table has to be done column-wise. The column must be present in the table, except when using the print only (-p) mode. Use db.execute or v.db.addcolumn to add new columns if needed.

## EXAMPLES

### Updating attribute tables

Upload category numbers to attribute table (used for new map):

    v.to.db map=soils type=centroid option=cat

Upload polygon areas to corresponding centroid record in the attribute table:

    v.to.db map=soils type=centroid option=area columns=area_size unit=h

Upload line lengths (in meters) of each vector line to attribute table (use v.category in case of missing categories):

    v.to.db map=roads option=length type=line columns=linelength units=me

Upload x and y coordinates from vector geometry to attribute table:

    v.to.db map=pointsmap option=coor columns=x,y

Upload x, y and z coordinates from vector geometry to attribute table:

    v.to.db map=pointsmap option=coor columns=x,y,z

Transfer attributes from a character column (with numeric contents) to a new integer column:

    v.db.addcolumn usa_income_employment2002 col="FIPS_NUM integer"
    v.to.db usa_income_employment2002 option=query columns=FIPS_NUM query_column=STATE_FIPS

Upload category numbers of left and right area, to an attribute table of boundaries common for the areas:

    # add categories for boundaries of the input vector map, in layer 2:
    v.category soils out=mysoils layer=2 type=boundary option=add
    # add a table with columns named "left" and "right" to layer 2 of the input
    # vector map:
    v.db.addtable mysoils layer=2 columns="left integer,right integer"
    # upload categories of left and right areas:
    v.to.db mysoils option=sides columns=left,right layer=2
    # display the result:
    v.db.select mysoils layer=2

Compute DL, the Fractal Dimension (Mandelbrot, 1982), of the boundary defining a polygon based on the formula:
`D = 2 * (log perimeter) / (log area):
`

    g.copy vect=soils,mysoils
    v.db.addcolumn mysoils col="d double precision"
    v.to.db mysoils option=fd column="d"

    g.region vector=mysoils res=50
    v.to.rast input=mysoils output=soils_fd type=area use=attr attribute_column=d
    r.colors map=soils_fd color=gyr

    d.mon wx0
    d.rast.leg soils_fd
    d.vect mysoils type=boundary

### Printing reports

Report x,y,z coordinates of points in the input vector map:

    v.to.db -p bugsites option=coor type=point

Report all area sizes of the input vector map:

    v.to.db -p soils option=area type=boundary units=h

Report all area sizes of the input vector map, in hectares, sorted by category number (requires GNU sort utility installed):

    v.to.db -p soils option=area type=boundary units=h | sort -n

Report all line lengths of the input vector map, in kilometers:

    v.to.db -p roads option=length type=line units=k

Report number of features for each category in the input vector map:

    v.to.db -p roads option=count type=line

## SEE ALSO

d.what.vect, db.execute, v.category, v.db.addtable, v.db.addcolumn, v.db.connect, v.distance, v.report, v.univar, v.what

## REFERENCES

- Mandelbrot, B. B. (1982). The fractal geometry of nature. New York: W. H. Freeman.
- Xu, Y. F. & Sun, D. A. (2005). Geotechnique 55, No. 9, 691-695

## AUTHOR

Radim Blazek, ITC-irst, Trento, Italy
Line sinuousity implemented by Wolf Bergenheim

Last changed: $Date: 2017-01-29 15:04:26 +0100 (Sun, 29 Jan 2017) $