Simple example to read+edit KML files?

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frdt...@gmail.com

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Aug 26, 2020, 10:32:51 AM8/26/20
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Hello,

My knowledge of Ruby and XML is shallow.

I need to read/edit KML files, but am going nowhere fast :-/

================================================
require 'nokogiri'

#============ 1. Read KML
xmldoc = Nokogiri::XML(File.read "track.kml")

#============ 2. Set //Document/Name to filename, regardless if it exists or not
puts File.basename(__FILE__)

#============ 3. Loop through all <Placemark>…</Placemark> blocks
xmldoc.xpath("//Placemark").each do |link|
  #If current placemark contains LineString, add <some>element</some> to block
end
#============ #4 Write KML to new file.

================================================ 
 
Would someone have a small example I could use to get started?

Thank you.

Mike Dalessio

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Aug 26, 2020, 10:44:48 AM8/26/20
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Hi,

Thanks for reaching out, and I'm sorry that Nokogiri doesn't make it more obvious how to do this kind of document manipulation.

Can I suggest that you start by looking at:


If those aren't sufficient, can you help me understand where you're blocked by asking more-specific questions?


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frdt...@gmail.com

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Aug 26, 2020, 10:59:04 AM8/26/20
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Thanks. I read those, but I'm stuck.

1. How to read and loop through all <Placemark> block?

xmldoc.xpath("//Placemark").each do |link|
  #If current placemark contains LineString, add <some>element</some> to block
end

2. How to check if the current block holds a <LineString> element. If not; how to add one?

3. How to save what's in RAM into a new KML file?

I'm not clear at the difference between CSS and XPath, to give you an idea at where I stand.

Thank you.

frdt...@gmail.com

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Aug 26, 2020, 11:00:08 AM8/26/20
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" 2. How to check if the current block holds a <LineString> element. If not; how to add one? "
"2. How to check if the current block holds a <LineString> element. If not; how to add a child element <some>blah</some>? 

Mike Dalessio

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Aug 26, 2020, 11:34:02 AM8/26/20
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OK, thanks for clarifying. If you could provide a short example "input" XML snippet and the desired "output" XML snippet, that would help me to find a good path to get from one to the other ... is that something you're able to provide?

frdt...@gmail.com

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Aug 26, 2020, 12:24:08 PM8/26/20
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Sure. Here's what a KML file looks like:

<?xml version="1.0" encoding="UTF-8"?> <kml xmlns="http://www.opengis.net/kml/2.2"> <Document> <name>Document.kml</name> <Placemark> <name>Waypoint</name> <Point> <coordinates>-122.371,37.816,0</coordinates> </Point> </Placemark> <Placemark> <name>Track</name> <LineString> <coordinates>-0.376291,43.296237,199.75 -0.376299,43.296237,199.75</coordinates> </LineString> </Placemark> </Document> </kml> 

I simply need to insert/replace the document's name (if there's any), and add a <some>blah</some> to any Placemark block that contains a LineString. 

Mike Dalessio

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Aug 26, 2020, 1:41:50 PM8/26/20
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Here's an attempt to answer your questions with working code:


For posterity, the code looks like:

```
#! /usr/bin/env ruby

require "nokogiri"

xml = <<EOF
<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
  <Document>
    <name>Document.kml</name>
    <Placemark>
      <name>Waypoint</name>
      <Point>
        <coordinates>-122.371,37.816,0</coordinates>
      </Point>
    </Placemark>
    <Placemark>
      <name>Track</name>
      <LineString>
        <coordinates>-0.376291,43.296237,199.75 -0.376299,43.296237,199.75</coordinates>
      </LineString>
    </Placemark>
  </Document>
</kml>
EOF

doc = Nokogiri::XML(xml)

# 1. insert/replace the document's name. 
#
# First, let's find the name node.
# For explanation of why the "xmlns" is needed, check out:
# > https://nokogiri.org/tutorials/searching_a_xml_html_document.html#namespaces
# 
name_node = doc.at_xpath("/xmlns:kml/xmlns:Document/xmlns:name")

#
# You could also use a CSS query which will (mostly) ignore namespaces. This is exactly the same search.
#
name_node = doc.at_css("kml > Document > name")

#
# Modify the contents of the <name/> node
#
name_node.content = "New Document Name"


# 2. a. find all Placemark blocks that contain a LineString
#
# I can think of two ways to do this. The first way is to search for
# LineString nodes within a Placemark node, and then get those nodes' parents:
placemarks_with_linestring = doc.xpath("//xmlns:Placemark/xmlns:LineString").map(&:parent)

# This first approach would work with CSS as well:
placemarks_with_linestring = doc.css("Placemark > LineString").map(&:parent)

# The second approach is to just use an XPath query to express that
# you want Placemarks that contain a LineString:
placemarks_with_linestring = doc.xpath("//xmlns:Placemark[xmlns:LineString]")

# Then you can add a new child node to that Placemark:
placemarks_with_linestring.each do |placemark|
  # the string passed into add_child is parsed just like any other XML fragment
  placemark.add_child "<some>blah</some>"
end

# The end result:
puts doc.to_xml
# >> <?xml version="1.0" encoding="UTF-8"?>
# >> <kml xmlns="http://www.opengis.net/kml/2.2">
# >>   <Document>
# >>     <name>New Document Name</name>
# >>     <Placemark>
# >>       <name>Waypoint</name>
# >>       <Point>
# >>         <coordinates>-122.371,37.816,0</coordinates>
# >>       </Point>
# >>     </Placemark>
# >>     <Placemark>
# >>       <name>Track</name>
# >>       <LineString>
# >>         <coordinates>-0.376291,43.296237,199.75 -0.376299,43.296237,199.75</coordinates>
# >>       </LineString>
# >>     <some>blah</some></Placemark>
# >>   </Document>
# >> </kml>

# 3. Write this to a file
#
# Use normal Ruby idioms for opening a file and writing to it, and use #to_xml to serialize the doc:
File.open("output.kml", "w") do |file|
  file.write doc.to_xml
end
```

frdt...@gmail.com

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Aug 26, 2020, 7:24:24 PM8/26/20
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Thanks very much!

=============
require "nokogiri"

doc = Nokogiri::XML(File.read "input.kml")

#I'm only using a single namespace, so let's keep things simple
doc.remove_namespaces!

name_node = doc.at_xpath("/kml/Document/name")
name_node.content = "New Document Name"

placemarks_with_linestring = doc.xpath("//Placemark[LineString]")
placemarks_with_linestring.each do |placemark|
  placemark.add_child "<some>blah</some>"
end

puts doc.to_xml

File.open("output.kml", "w") do |file|
  file.write doc.to_xml
end
==============

Ouf ot curiosity, why is "<some>blah</some>" not lined up, has the </Placemark> trailing, and is there a way to remedy this?


8CE87A5E-DFB7-4DEB-8D8C-36784CFF480F.png

Mike Dalessio

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Aug 27, 2020, 3:23:43 PM8/27/20
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Hi,

> doc.remove_namespaces!

Please keep in mind that your output document will then not have any namespaces, which may be a problem for anybody consuming your edited KML file.

> why is "<some>blah</some>" not lined up

Whitespace is a text node like any other, though usually it carries no semantic meaning in XML documents. If you'd like to eliminate whitespace altogether, you could parse this way:

doc = Nokogiri::XML(xml) do |config|
  config.noblanks
end



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frdt...@gmail.com

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Aug 28, 2020, 5:23:57 AM8/28/20
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Thank you very much.

Mike Dalessio

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Aug 28, 2020, 8:44:14 AM8/28/20
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Just as an aside, this question and use case are really well-suited to be an example in the Nokogiri tutorials. I've created an issue to drive that addition: https://github.com/sparklemotion/nokogiri.org/issues/29

Thanks again for asking!

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