topic Re: arcpy.Warp_management not performing as expected in Python Questions

arcpy.Warp_management not performing as expected

AnthonyCheesman1 — Mon, 04 Jul 2016 06:52:10 GMT

Hi - I'm unsure whether this is a Python question or a geoprocessing question. I'll post it here and see how we go.

I am trying to use the 'warp' function in a rather novel way. I have a heap of (~800) individual QR codes, which I'd like to georeference so that they can be displayed as map features.

The QR codes are all the same size and I can use the same reference points, whereas the location to georeference the features 'to' (ie where I want them to sit on the map) will change from code to code. This location is determined by the location of a given point.

I've cobbled together a script to iterate through the codes, and the script runs correctly, but the output features aren't georeferenced at all.

I've also tried georeferencing a single feature via the Toolbox, but that doesn't perform as expected either.

The script (snippet of) I'm using is below:

ls_inputs = os.listdir(ws_inputs)

for i in ls_inputs:

if i[0:-4] in dict_mapcentroid:

print "Working on", i

input = os.path.join(ws_inputs, i)

cent_x = dict_mapcentroid[i[0:-4]][0]

cent_y = dict_mapcentroid[i[0:-4]][1]

target_pt = "'11.5 -158.5';'158.5 -11.5'"

source_pt = "'%s %s';'%s %s'" % (cent_x - 1750, cent_y - 1750, cent_x + 1750, cent_y + 1750)

output = os.path.join(ws_outputs, "georef_%s" % i)

arcpy.Warp_management(input, source_pt, target_pt, output, "POLYORDER1")

else:

print "Fail on", i

(dict_mapcentroid refers to a dictionary with the feature name as key, and the coordinates of the centroid as a [x,y] list)

(the reference points are bottom left and top right, the +/- 1750 allow for the scaling of the feature to appear 3 x 3cm at 1:250K)

Can anyone shed any light on why this wouldn't be working - or am I simply trying to use a tool in a manner in which it was not intended?

Thanks

Re: arcpy.Warp_management not performing as expected

DanPatterson_Retired — Mon, 04 Jul 2016 08:26:43 GMT

The help link has a reference to the minimum number of points required to 'warp' a raster to a new location. You have one, you need 6 for a first order. So the direct answer to your question is, you are using it for the wrong purpose

Warp—Help | ArcGIS for Desktop

There is

Shift—Help | ArcGIS for Desktop

which will get a raster to a new location, however what you are moving would need a known cell size to make it useful.

So if what you are moving, in the form of a raster, has a known cell size and just needs to be moved, you 'shift' it, if there is distortion or the cell size is unknown or just a big picture, you need to warp it. If it is vector data, you need to use the Spatial Adjustment tool

Re: arcpy.Warp_management not performing as expected

AnthonyCheesman1 — Mon, 04 Jul 2016 11:06:38 GMT

Thanks Dan for the information and your prompt reply (as always).

The data I am attempting to use is in .png format and does need to be scaled as well as moved, hence the warp function being used.

When you say 6 control points - do you mean 3 on the source and 3 on the target? I'm using 2 (or 4, depending on the interpretation). The (p+1)(p+2)/2 formula on the help page tells me 3 points for a 1st order polynomial, so I'll give that a crack tomorrow.

Hopefully an extra control point fixes this!

Re: arcpy.Warp_management not performing as expected

DanPatterson_Retired — Mon, 04 Jul 2016 13:11:06 GMT

It is strongly suggested to use more than the minimum number of links.

I would go wild and whip in a few more... it is the placement that is important as well... which may seem obvious, but based on lab assignments I have had to deal with... less so than one would imagine

Re: arcpy.Warp_management not performing as expected

AnthonyCheesman1 — Tue, 05 Jul 2016 00:36:15 GMT

Success! 4 reference points has solved the problem.

Thanks for the help Dan.