[GRASS-user] Change column data format from double precision to integer

[Micha Silver]

Hi Rich


On 10/5/21 7:59 PM, Rich Shepard wrote:
> A vector map's database table has a column named z_int and data type of
> double precision as presented in the map's attribute table. It was 
> defined
> as an int in the v.in.ascii command. How do I change the data type to 
> int?
>

AFAIK coordinates in GRASS (including the z coord) are always DOUBLE. 
The v.in.ascii function reads the x,y,z columns as DOUBLE.

If you declare the columns parameter in v.in.ascii, these attribute 
columns are created as DOUBLE.

I checked the OGC specs for vector data and did not find any mention 
that coordinates should be DOUBLE, but that's usually the case in most 
GIS, to the best of my knowledge.


Having said that, you can create a new attribute column, and populate it 
with integer values from the z coordinate, with the CAST sqlite3 
function. Consider:


echo "1,1,1" | v.in.ascii -z input=- output=p1 separator=comma x=1 y=2 z=3

v.report p1 option=coor
cat|x|y|z
1|1|1|1

# However, no database connection defined:

v.info -c p1
ERROR: Database connection for map <p1> is not defined in DB file


# v.to.db creates new columns as DOUBLE

v.to.db p1 option=coor column="east,north,elev"

v.info -c p1
Displaying column types/names for database connection of layer <1>:
INTEGER|cat
DOUBLE PRECISION|east
DOUBLE PRECISION|north
DOUBLE PRECISION|elev


# Now add a new INTEGER column

v.db.addcolumn p1 column="z_int INTEGER"

v.db.update p1 column=z_int query_col="CAST(elev as INTEGER)"

v.info -c p1
Displaying column types/names for database connection of layer <1>:
INTEGER|cat
DOUBLE PRECISION|east
DOUBLE PRECISION|north
DOUBLE PRECISION|elev
INTEGER|z_int


I think that is what you wanted.



> The only option offered in that window is to delete a column by
> right-clicking on it. I assume there's a way within GRASS of changing the
> data type.
>
> TIA,
>
> Rich
>
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-- 
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918