[GRASS-user] Problems getting unique categories with v.category

[Markus Metz]

On Thu, Oct 19, 2017 at 8:38 PM, Eric Patton <thepattonian at gmail.com> wrote:
>
> Hi,
>
> I have created a point vector from a coastline using v.to.points:
>
> v.to.points input=CanCoast_Shoreline  type=line
output=CanCoast_Shoreline_points use=vertex --v
>
> This works fine, and the next thing I want to do is upload raster slope
values from a DEM to the attribute table of the points vector using
v.what.rast, but I get tons of warnings like these:
>
> WARNING: Multiple points (12) of category 16, value set to 'NULL'
>
> I assume this is telling me the points have duplicate categories?

Exactly. From the manual of v.to.points:
"The output is a vector map with 2 layers. Layer 1 holds the category of
the input features; all points created along the same line have the same
category, equal to the category of that line. In layer 2 each point has its
unique category"

The solution is to upload values to layer 2 with v.what.rast layer=2
>
> So to get around this, I tried recreating the points, this time without
an attribute table, using v.to.points -t, then adding the table and
categories manually:

With v.to.points -t, the category values are already there, but not the
tables.

HTH,

Markus M

>
> v.to.points input=CanCoast_Shoreline  type=line
output=CanCoast_Shoreline_points use=vertex -t --v --o
> v.db.addtable map=CanCoast_Shoreline_points columns="Slope_percent
integer"
> v.category input=CanCoast_Shoreline_points option=add
output=CanCoast_Shoreline_points_with_cat
>
> But running v.what.rast still gives me the same error as before.
>
> I ran v.categories in report mode to see what is going on:
>
> v.category input=CanCoast_Shoreline_points_with_cat option=report
> Layer/table: 1/CanCoast_Shoreline_points_with_cat
> type       count        min        max
> point     105427          1       1692
> line           0          0          0
> boundary       0          0          0
> centroid       0          0          0
> area           0          0          0
> face           0          0          0
> kernel         0          0          0
> all       105427          1       1692
> Layer: 2
> type       count        min        max
> point     105427          1     105427
> line           0          0          0
> boundary       0          0          0
> centroid       0          0          0
> area           0          0          0
> face           0          0          0
> kernel         0          0          0
> all       105427          1     105427
>
> Where did the two layers come from? I thought v.to.points -t *wasn't*
supposed to make an attribute table. And the fact that the 'max' value
reported above is '1692' seems to be a hangover from the line features they
were created from.
>
> In this case, I don't care about the IDs or categories of the lines the
points originated from; I want unique categories for the 105427 points that
were made so I can then successfully run v.what.rast and upload my DEM
slope values.
>
> It's been a while since I've used these tools, so I'm a little rusty. Is
there a way to do this?
>
>
> ~ Eric.
>
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