Centroid and beginning/ending vertices (nodes) with Arcade (and other questions)

[ToddFagin]

Greetings Attribute Rules community.

Though a long-time GIS user, I am new to both Attribute Rules and Arcade.

I have what I believe to be a fairly simply problem and hope someone can help.

I have a colleague who would like me to develop a field map for her that includes a linear feature called transects. She then needs to derive the starting vertex X,Y, centroid X,Y, and ending vertex X,Y of the newly created feature. I figured that an attribute rule that automatically calculates it would be great.

The centroid rule seemed pretty easy--Round(Centroid($feature).X, 6) and Round(Centroid($feature).Y, 6) to calculate the X and Y centroids, respectively. Hurray. Well, no.

It works, but it appears to be calculating the centroid for the boundary geometry of the feature rather than the actual centroid of the linear feature (i.e. actually a point ON the line). Suggestions?

As for the beginning and ending vertices X,Y, I am at a loss. I can use something like Round(Geometry($feature).Y, 6) to find the coordinate of a point, but how can I implement this for a vertex in a line or polygon?

Lastly, and this is more philosophical, I suppose. Is there a way to calculate a spatial relationship with Arcade? (Again, being new to this, this may be a no brainer). For instance, if I plot a point that is within the geometry of a polygon, I would like to do some equivalent of a spatial join to assign the point attribute(s) from the polygon.

Thanks in advance.

[JohannesLindner · Accepted solution]

It works, but it appears to be calculating the centroid for the boundary geometry of the feature rather than the actual centroid of the linear feature (i.e. actually a point ON the line). Suggestions?

Hmmm. I believe snapping the "false" centroid to the closest line segment would not yield the "correct" centroid, right?

As for the beginning and ending vertices X,Y, I am at a loss

// the geometry of line features has a "paths" attribute, which is a list of
// lists of points (multiple line segments)
// for polygons, the attribute is named "rings".
var first_point = Geometry($feature).paths[0][0]
var last_point = Geometry($feature).paths[-1][-1]

Is there a way to calculate a spatial relationship with Arcade? (Again, being new to this, this may be a no brainer). For instance, if I plot a point that is within the geometry of a polygon, I would like to do some equivalent of a spatial join to assign the point attribute(s) from the polygon.

// load the polygons using one of the FeatureSetBy* functions
var fs_polygons = FeatureSetByName($datastore, "PolygonFC")
// find polygons intersecting the $feature
var intersecting_polygons = Intersects(fs_polygons, $feature)
// no polygons found -> return early
if(Count(intersecting_polygons) == 0) { return null }
// intersecting_polygons is a feature set, we want to access the attributes 
// of polygon features, so we have to extract one of them. 
// easiest, but not always appropriate, way is to just grab the first one.
var polygon = First(polygons)
// return some attribute
return polygon.Attribute

If you didn't know yet, the Arcade functions and classes are pretty well documented:

Arcade functions:
Function Reference | ArcGIS Arcade | ArcGIS Developer

Arcade classes:
Type System | ArcGIS Arcade | ArcGIS Developer

[ToddFagin]

I appreciate your assistance on this. I see I still have a lot to do to completely wrap my head around Attribute Rules and Arcade.