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What is a memory vector layer and how does it relate to the vector layer?

Background info:

I have a volunteer working with my organization, because of financial reasons we have him using QGIS. He is relatively new to GIS so has some questions. Being a ArcGIS user I can field most of his questions, as (it seems) the 2 programs have a lot of similarities.

  • A memory layer is like any other vector layer but it doesn’t exist on disk and disappears when QGIS exits. If needed, it can be saved to disk. – gene Dec 22 '14 at 16:26
  • so is it referenced or linked to "its" vector layer? – Mapsy Daisy Dec 22 '14 at 16:30
  • no, it is a real "temporary" vector layer, independent of the others – gene Dec 22 '14 at 17:50
  • What is the purpose of it? – Mapsy Daisy Dec 22 '14 at 18:23
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    I would say one example using an in-memory layer would be if it was an intermediate step in processing to produce a final output that would be saved. Meaning you only need the layer temporarily to do some process but once you have the desired output you no longer need the intermediate layer. – landocalrissian Dec 22 '14 at 18:33
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In QGIS, a memory layer is like a normal vector layer, but it is created to be temporary. You can create memory layers by using the new memory layer plugin.

New memory layer plugin

Notice that the layer will only exists within your current open project. Once the project is closed, IT WILL DISAPPEAR.

You can workaround this with another plugin called memory layer saver that, when the project is saved, will save all your project memory layers in mldata file next to it. This way when you reopen your project all your temporary layers will be there.

Memory layer saver

You can also make a memory layer permanent (to any format) by using the save as tool.

The current QGIS version currently in development (2.7 master) has integrated the memory layer plugin in it's core functions, but with a different name, Scratch layers.

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