Surface buffer

GIS
The current state of a volcano is extremely variable with time and it has no meaning to define fixed risk zones around volcanoes (e
Author

Benny Istanto

Published

April 22, 2019

The current state of a volcano is extremely variable with time and it has no meaning to define fixed risk zones around volcanoes (e.g. “from 5 to 10km from the crater”)

Currently, during the emergency situation in volcano eruption case, most of decision maker with help from the technical team define the safe zone from the crater without considering the surface (blue line in below picture).

Figure 1 . Distance from a location

Figure 1 . Distance from a location

Despite there is no default distance for volcanic safe zone, but it is important to calculate the true distance (considering the surface) from the crater — red line on above picture. For saving more lives and better preparedness.

Using GIS technology, we can calculate the true distance very easily. In the ESRI GeoNet forum, there has been discussions on how to calculate a surface buffer using ArcPy. What we need is a coordinate of selected location and the surface condition from the surrounding areas, we can use free elevation data from SRTM and can be downloaded from this site: 30-Meter SRTM Tile Downloader.

Figure 2. 12 km from the peak of Gunung Agung, Bali - Indonesia

Figure 2. 12 km from the peak of Gunung Agung, Bali - Indonesia

Picture above is the result, red line is the true distance of 12km from the crater considering the topographic condition. You can try to calculate with your own data using below script from Xander Bakker of ESRI Colombia below:

import arcpy 
import os 
import math 
 
# get SA license 
arcpy.CheckOutExtension(“Spatial”) 
 
# settings, input data 
max_dist = 12000 
steps = 360 # 1 point each degree 
ws = r”Z:\Temp\SurfaceBuffer\Bali_UTM.gdb” 
dem = r”Z:\Temp\SurfaceBuffer\Bali_UTM.gdb\DEM” 
fc_pnt= r”Z:\Temp\SurfaceBuffer\Bali_UTM.gdb\Mt_Agung” 
 
# output and intermediate names 
slp_per_name = “slope_perc” 
fc_buf_name = “pnt_buf” 
fc_linepnt_name = “rad_pnt” 
fc_linepntdem_name = “rad_pnt_dem” 
fc_linepntdemsel_name = “rad_pnt_dem_sel” 
fc_buf3D_name = “buf_3D” 
 
# internal settings 
arcpy.env.workspace = ws 
arcpy.env.overwriteOutput = True 
# ws_mem = “IN_MEMORY” 
fld_rasval = “RASTERVALUE” 
 
# use cellsize as interval 
##ras_dem = arcpy.Raster(dem) 
##cellw = ras_dem.meanCellWidth 
##cellh = ras_dem.meanCellHeight 
##pixsize = int((cellw + cellh) / 2) 
 
#overwrite precision along line 
pixsize = 30.92336053
steps2 = max_dist / pixsize 
 
# Get point from input fc 
with arcpy.da.SearchCursor(fc_pnt, (“SHAPE@XY”)) as curs: 
 for row in curs: 
 pnt_cc = arcpy.Point(row[0][0],row[0][1]) 
 break 
 
# Createa normal buffer on point with max distance 
fc_buf = os.path.join(ws, fc_buf_name) 
arcpy.Buffer_analysis(fc_pnt, fc_buf, “{0} METERS”.format(max_dist)) 
 
# get buffer geometry 
with arcpy.da.SearchCursor(fc_buf, (“SHAPE@”)) as curs: 
 for row in curs: 
 geom = row[0] 
 break 
 
# get outline 
polyline = geom.boundary() 
length = polyline.length 
interval = length / steps 
 
# create new featureclass to hold points on lines 
sr = arcpy.Describe(fc_pnt).spatialReference 
arcpy.CreateFeatureclass_management(ws, fc_linepnt_name, “POINT”, “”, “DISABLED”, “DISABLED”, sr) 
fc_linepnt = os.path.join(ws, fc_linepnt_name) 
 
# add columns 
fld_id = “LineID” 
fld_dist = “Distance” 
fld_slpper = “SlopePerc” 
fld_dist3D = “Dist3D” 
fld_id2 = “LinePntID” 
 
arcpy.AddField_management(fc_linepnt, fld_id, “LONG”) 
arcpy.AddField_management(fc_linepnt, fld_dist, “DOUBLE”) 
arcpy.AddField_management(fc_linepnt, fld_slpper, “DOUBLE”) 
arcpy.AddField_management(fc_linepnt, fld_dist3D, “DOUBLE”) 
arcpy.AddField_management(fc_linepnt, fld_id2, “TEXT”, 25) 
 
flds = (“SHAPE@”, fld_id, fld_dist, fld_id2) 
with arcpy.da.InsertCursor(fc_linepnt, flds) as curs: 
 # loop over boundary 
 for i in range(0, steps): 
 d = i * interval 
 line_id = i 
 
 # extract point on buffer outline 
 pnt = polyline.positionAlongLine(d, False) 
 
 # create line from center to pnt on line 
 arr = arcpy.Array() 
 arr.removeAll 
 arr.add(pnt_cc) 
 arr.add(pnt.firstPoint) 
 line = arcpy.Polyline(arr) 
 
 # extract points on line 
 for j in range(0, int(steps2)): 
 dist = int(j * pixsize) 
 pnt2 = line.positionAlongLine(dist, False) 
 
 # add to fc in mem with line ID and eucledian distance 
 val_id = “{0}_{1}”.format(line_id, dist) 
 curs.insertRow((pnt2, line_id, dist, val_id)) 
 
 
# extract DEM values 
fc_linepntdem = os.path.join(ws, fc_linepntdem_name) 
arcpy.sa.ExtractValuesToPoints(fc_linepnt, dem, fc_linepntdem, “INTERPOLATE”, “VALUE_ONLY”) 
 
# create nested dictionary for update 
##flds = “;”.join([fld_id, fld_dist, fld_rasval, fld_slpper, fld_dist3D]) 
##rows = arcpy.SearchCursor(fc_linepntdem, fields=flds, sort_fields=”{0} A; {1} A”.format(fld_id, fld_dist)) 
##dct_vals = {“{0}_{1}”.format(r.getValue(fld_id), r.getValue(fld_dist)):[r.getValue(fld_id), r.getValue(fld_dist), r.getValue(fld_rasval), -1, -1] for r in rows} 
flds = (fld_id, fld_dist, fld_rasval, fld_id2) 
dct_vals = {r[3]:[r[0], r[1], r[2], -1, -1] for r in arcpy.da.SearchCursor(fc_linepntdem, flds)} 
 
# create list of sort order data 
flds = “;”.join([fld_id, fld_dist, fld_id2]) 
rows = arcpy.SearchCursor(fc_linepntdem, fields=flds, sort_fields=”{0} A; {1} A”.format(fld_id, fld_dist)) 
lst_vals_ids = [r.getValue(fld_id2) for r in rows] 
 
# save for each line the id of the outside point 
dct_res = {} 
# do some magic… 
for val_id in lst_vals_ids: 
 if val_id in dct_vals: 
 lst = dct_vals[val_id] 
 line_id = lst[0] 
 if lst[1] == 0: 
 # start of line 
 lst[3] = 0 
 lst[4] = 0 
 h0 = lst[2] 
 dct_vals[val_id] = lst 
 distcum = 0 
 else: 
 # other points 
 h1 = lst[2] 
 h_dif = abs(h1-h0) 
 dist3D = math.sqrt((h_dif**2) + (pixsize**2)) 
 slope = h_dif * 100.0 / pixsize 
 distcum += dist3D 
 lst[3] = slope 
 lst[4] = distcum 
 dct_vals[val_id] = lst 
 h0 = h1 
 if distcum <= max_dist: 
 dct_res[line_id] = val_id 
 
# write slope and distance cum to featureclass 
flds = (fld_id, fld_dist, fld_slpper, fld_dist3D) 
with arcpy.da.UpdateCursor(fc_linepntdem, flds) as curs: 
 for row in curs: 
 val_id = “{0}_{1}”.format(row[0], row[1]) 
 if val_id in dct_vals: 
 lst = dct_vals[val_id] 
 row[2] = lst[3] 
 row[3] = lst[4] 
 curs.updateRow(row) 
 
# select points within max distance 
arcpy.MakeFeatureLayer_management(fc_linepntdem, “lyr”) 
 
# define where clause based on list of id’s 
txt_ids = (str(dct_res.values()))[1:-1] 
txt_ids = txt_ids.replace(‘u’,’’) 
where = “{0} IN ({1})”.format(arcpy.AddFieldDelimiters(fc_linepntdem, fld_id2), txt_ids) 
arcpy.SelectLayerByAttribute_management(“lyr”, “NEW_SELECTION”, where) 
 
# copy to new fc (for debug) 
fc_linepntdemsel = os.path.join(ws, fc_linepntdemsel_name) 
arcpy.CopyFeatures_management(“lyr”, fc_linepntdemsel) 
 
# create convexHull on points for 3D buffer 
# this will give erroneous results on concave parts 
##cnt = 0 
##with arcpy.da.SearchCursor(fc_linepntdemsel, (“SHAPE@”)) as curs: 
## for row in curs: 
## if cnt == 0: 
## pntg = row[0] 
## else: 
## pntg_tmp = row[0] 
## pntg = pntg.union(pntg_tmp) 
## cnt += 1 
## 
##pol = pntg.convexHull() 
 
# create polygon manually, sort points on line_id 
arr_pol = arcpy.Array() 
fld_shape = arcpy.Describe(fc_linepntdemsel).shapeFieldName
flds = “;”.join([fld_id, fld_shape]) 
rows = arcpy.SearchCursor(fc_linepntdemsel, fields=flds, sort_fields=”{0} A”.format(fld_id)) 
cnt = 0 
for row in rows: 
 if cnt == 0: 
 end_pntg = row.getValue(fld_shape) 
 end_pnt = end_pntg.firstPoint 
 
 pntg = row.getValue(fld_shape) 
 pnt = pntg.firstPoint 
 arr_pol.add(pnt) 
 cnt += 1 
 
arr_pol.add(end_pnt) 
del row 
del rows 
 
pol = arcpy.Polygon(arr_pol) 
 
# write polygon to output fc 
fc_buf3D = os.path.join(ws, fc_buf3D_name) 
arcpy.CreateFeatureclass_management(ws, fc_buf3D_name, “POLYGON”, “”, “DISABLED”, “DISABLED”, sr) 
with arcpy.da.InsertCursor(fc_buf3D, “SHAPE@”) as curs: 
 row = (pol,) 
 curs.insertRow(row) 
 
# return SA license 
arcpy.CheckInExtension(“Spatial”)

You need to adjust the distance in line 9 (in meters) and working directory in line 11–13. And then you can paste the code into python GUI inside ArcGIS, or run via command prompt.

Good luck!

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