I have a problem when creating polygon features with arcpy. I want to save rounded X and Y vertices for example to two decimal places. Example:
I have coordinates given on two decimal places. I am expecting rounding error due to limitations in precision of float data type.
Lets say we are working with projected coordinate system with base units of 1 Meter.
Example script (using dummy coordinate values):
import arcpy
point_coord = arcpy.Point(1.10, 2.13)
point = arcpy.PointGeometry(point_coord)
linestring_coords = [arcpy.Point(1.10, 2.13), arcpy.Point(2.44, 5.12)]
ls_array = arcpy.Array(linestring_coords)
linestring = arcpy.Polyline(ls_array)
poly_coords = [arcpy.Point(1.10, 2.13), arcpy.Point(2.44, 5.12), arcpy.Point(0.32, 4.2), arcpy.Point(1.10, 2.13)]
p_array = arcpy.Array(arcpy.Array(poly_coords))
poly = arcpy.Polygon(p_array)
print("Constructed point: {}".format(point.WKT))
print("Constructed polyline: {}".format(linestring.WKT))
print("Constructed polygon: {}".format(poly.WKT))
Result:
Constructed point: POINT (1.1000000000000001 2.1299999999999999)
Constructed polyline: MULTILINESTRING ((1.1000000000000001 2.1299999999999999, 2.4399999999999999 5.1200000000000001))
Constructed polygon: MULTIPOLYGON (((1.10009765625 2.130126953125, 2.44012451171875 5.1201171875, 0.32012939453125 4.2000732421875, 1.10009765625 2.130126953125)))
As said I am expecting rounding errors on for the values in point and polyline. But error on polygon coordinates is a lot bigger than 10e-10. If we say that base projection units are in meters that brings an error of 0.1 mm which is a lot in some surveying applications. I know that this error can be virtually rounded to 2 decimal places in arcmap or some other, but if I want to take these coordinates programically and perform some calculations the error grows and stays in data.
Is there a way to enforce rounding to values of coordinates for polygon features. How can this be the case in software meant for managing acurrate and high quality data.
I know for a fact, that some libraries outside arcgis ecosystem round coordinates normally for polygons (like point and polyline above).
How can I achieve same 'precision' as when creating polylines but with Polygons.
Thanks in advance.
There are issues if you don't use a defined coordinate system for your data. I didn't see one specified.
specify it when creating the Polygon object
poly = arcpy.Polygon(arcpy.Array(aa), SR)
where aa is a list or array of points and SR is a spatial reference object (name, of code is also fine)
Thank you for your fast anwser!!
I can see what you mean. I didn't think of defining coordinate system for each geometry object since feature class I'm inserting in (in other solution) has spatial reference defined. This fixed the accuracy a lot but it is still not the same as with polyline if we compare first two vertices.
Example:
import arcpy
sr = arcpy.SpatialReference(3857) #projected coordinate system
point_coord = arcpy.Point(1.10, 2.13)
point = arcpy.PointGeometry(point_coord, sr)
linestring_coords = [arcpy.Point(1.10, 2.13), arcpy.Point(2.44, 5.12)]
ls_array = arcpy.Array(linestring_coords)
linestring = arcpy.Polyline(ls_array, sr)
poly_coords = [arcpy.Point(1.10, 2.13), arcpy.Point(2.44, 5.12), arcpy.Point(0.32, 4.2), arcpy.Point(1.10, 2.13)]
p_array = arcpy.Array(arcpy.Array(poly_coords))
poly = arcpy.Polygon(p_array, sr)
print("Constructed point: {}".format(point.WKT))
print("Constructed polyline: {}".format(linestring.WKT))
print("Constructed polygon: {}".format(poly.WKT))
Results:
Constructed point: POINT (1.1000000000000001 2.1299999999999999)
Constructed polyline: MULTILINESTRING ((1.1000000000000001 2.1299999999999999, 2.4399999999999999 5.1200000000000001))
Constructed polygon: MULTIPOLYGON (((1.1000000014901161 2.1299999989569187, 2.4400000013411045 5.1200000010430813, 0.32000000029802322 4.1999999992549419, 1.1000000014901161 2.1299999989569187)))
Quick follow up question:
How come that polylines and points give right accuracy even without SR defined since they are (from my view) build with same underlying object (arcpy.Point, arcpy.Array)?
Polyline—ArcPy classes | ArcGIS Desktop
The Geometry, Polygon, Polyline, PointGeometry, MultiPoint classes all have a spatial reference... Point does not, nor does Array
Explore the ArcPy classes for more goodies