Each vector layer has an associated attribute table that contains one record (row) for each individual geographic feature in the layer. The attribute table typically contains multiple attribute data fields (columns) describing the features. Each raster layer has an attribute table that contains one record for each raster category value. (All the raster cells in a particular attribute category are treated as a single feature, even if they are geographically dispersed.) Access a layer's attribute table by right-clicking on it and choosing 'Open Attribute Table.' You can create new data fields in the 'Options' menu. Field types include short and long integers (no digits to the right of the decimal point!), single- or double-precision numbers (including digits to the right of the decimal point) and text. If you right-click on a field name, you can sort the records according to that field, summarize the data by field value, calculate new field values or extract summary statistics for that field. You will often want to focus your analysis on specific features in a vector layer, or specific cell categories in a raster. There are multiple ways to select individual features or sets of features from the map or attribute table: Briefly, a projection translates spherical (lat-lon) coordinates to a planar coordinate system such as UTM or State Plane. Each layer will typically have its own coordinate system specified. One of the nice things about the Arc data frame is that it can superimpose unprojected (lat-lon) and projected layers in various coordinate systems correctly. But be careful about calculating distance or area measures based on unprojected features, even if they are displayed in a projected coordinate system. The first lab shows how such errors can arise. To create a new (projected) version of a layer in the coordinate system of the dataframe, use Data--Export Data to create a copy of the layer, specifying that the output should be in the data frame's coordinate system. (This is quicker than using Arc's formal reprojection tools.) Source.


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Last Modified: April 23, 2016 @ 7:05 am