[GIS] a memory vector layer in QGIS

memory-layerqgisvector

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.

Best Answer

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.

Related Solutions

[GIS] Creating new empty memory layer with fields scheme from other layer in QGIS

Here's some quick-and-dirty Python code that works on a simple shapefile layer:

# Get the currently selected layer
inLayer = iface.activeLayer()

# Get its list of fields
inFields = inLayer.dataProvider().fields()

# Convert its geometry type enum to a string we can pass to
# QgsVectorLayer's constructor
inLayerGeometryType = ['Point','Line','Polygon'][inLayer.geometryType()]

# Convert its CRS to a string we can pass to QgsVectorLayer's constructor
inLayerCRS = inLayer.crs().authid()

# Make the output layer
outLayer = QgsVectorLayer(inLayerGeometryType + '?crs='+inLayerCRS, \
    inLayer.name() + u'_copy', \
    'memory')

# Copy the fields from the old layer into the new layer
outLayer.startEditing()
outLayer.dataProvider().addAttributes(inFields.toList())
outLayer.commitChanges()

# Add it to the map
QgsMapLayerRegistry.instance().addMapLayer(outLayer)

I load up a simple shape file, select that layer in the Layers box, paste the above code into the Python console, and a new layer appears with the same attributes.

The key parts here (I'm guessing from your question) are the lines that get the list of fields from the input layer (inLayer.dataProvider().fields(), documented here) and add those same fields to the output layer (outLayer.dataProvider().addAttributes(inFields.toList()), documented here)

[GIS] Is raster lossy and vector not lossy

These definitions you're using are not accurate:

not lossy (scale up or down, doesn't matter)

lossy (or have the potential for being lossy if attempted to scale up)

"Scale up or down, doesn't matter" is a defining characteristic of vector formats and has nothing to do with "lossy" vs "lossless". Similarly, how a raster looks when it's scaled up has nothing to do with whether it's lossless or lossy, it looks different when scaled up because rasters are based on cells(pixels) and scaling them up makes the pixels bigger.

The terms "lossless" and "lossy" apply to data compression techniques. So rasters can be either lossless or lossy depending on the format and the compression scheme used. A TIFF file, for example, can be created with no compression at all, lossless compression using LZW or PackBits, or lossy compression with JPEG. The Esri GRID raster format is compressed using lossless compression and the format doesn't support lossy compression at all.

Vector formats are not "lossy" in the same sense a JPEG is. The person you're quoting is stretching the definition of "lossy" to include things like line smoothing, which does have the effect of making a vector dataset smaller but isn't really a data compression scheme like JPEG is.

Best Answer

Related Solutions

[GIS] Creating new empty memory layer with fields scheme from other layer in QGIS

[GIS] Is raster lossy and vector not lossy

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