Tips on Importing Files to ArcGIS

ArcGIS assumes that any map layer is georeferenced to a particular location on the earth's surface. As powerful and universal an approach as this may be for combining spatial data from different sources, many kinds of spatial analysis do not require georeferencing. Such analyses may be carried out with data from the Comparative Archaeology Database irrespective of the availability or precision of georeferencing information. Subjects of such analysis include spatial clustering or dispersion, centralization, settlement hierarchy, network relationships, similarity of multiple spatial distributions, and many others. Most of the regional datasets in the Comparative Archaeology Database also include information on such environmental elements as topography, hydrology, soils, and other resources. These different map layers within a dataset are properly referenced to each other, even for datasets collected before attention to precise georeferencing was common or even possible for some regions. These sets of geographic information are internally consistent and analyzable on their own terms without need of georeferencing. Some GIS programs (including AutoCAD Map, GRASS, Idrisi and others) make it easy to carry out analyses in whatever internally consistent coordinate system is convenient, irrespective of georeferencing. Queries from users of ArcGIS, however, reveal that many are stymied by ArcGIS's insistence that all map layers be georeferenced. This issue can be resolved in ArcGIS, as discussed below. Georeferencing can also be accomplished even when georeferencing metadata are not available, also discussed below. These notes are not intended to substitute for ArcGIS documentation but simply to call attention to relevant ArcGIS tools that some users may not be aware of.

A .dxf file, for example, can be imported to ArcGIS by dragging it from the Catalog pane or using the Add Data shortcut. Ignore the ArcGIS warning that spatial information is missing and proceed. The AutoCAD entities now appear under the file Group Layer in the ArcGIS Table of Contents. AutoCAD Text entities appear as ArcGIS Annotation features; AutoCAD Point entities appear as ArcGIS Point features; AutoCAD Polyline entities appear as ArcGIS Polyline features; and AutoCAD Closed Polyline entities appear as ArcGIS Polygon features. The Attribute Tables for the ArcGIS features contain variables corresponding to various properties of the AutoCAD entities, including, for example, Layer, Color, and others.

If georeferencing metadata are available, this information can be used to set the properties of the ArcGIS Data Frame. This will likely require specifying the coordinate system, the base datum, and the map units. ArcGIS, for example, is one of several GIS programs that are quite persistent about using meters as the map units for the UTM projection system, whereas other software (including AutoCAD Map, GRASS, and Idrisi) allows more flexibility and UTM-based datasets may often use kilometers as the basic map unit.

ArcGIS provides excellent tools for georeferencing maps for which georeferencing metadata are not available or seem to be inaccurate. These tools are found on the Georeferencing and Spatial Adjustment toolbars. The most powerful and flexible tools allow establishing Control Points on the ungeoreferenced map and telling ArcGIS what the real-world coordinates of those control points are in a known coordinate system. Specific locations of sites, rivers, highways, modern towns, etc. are often included in a GIS dataset. The coordinates of these places can be established, in the UTM system or Lat/Long, based on the WGS84 datum, by finding them in a source such as Google Earth. To the extent that the coordinates of a number of control points can be established accurately in this way, georeferencing may have sufficient precision to permit overlays of georeferenced data from other sources. In any case of overlaying data from different sources, the wise analyst insists on visual verification of the accuracy of the spatial match. (Just how well do the locations of rivers, highways, or other identifiable features in both map layers match once they are georeferenced and overlaid?)

If the intended analysis does not involve overlays of spatial data from other sources, but only requires internal consistency within a single dataset, then it may be convenient in ArcGIS to locate the dataset in its approximate location in the world, quickly using the above approach. Great investment in achieving high precision is not necessary. As long as all imported elements of an internally consistent GIS dataset are treated in the same way, the internal consistency will be maintained.

Once spatial data are imported to ArcGIS, a Shapefile can be created with the Export Data option. If the Attribute Table containing the AutoCAD properties of the entities is no longer attached to them, it can be reattached, for example with the Feature to Point tool in the Data Management section of the ArcToolbox.

In a .dxf file, the identifiers of polygons often take the form of AutoCAD Text entities whose Insertion Points are located inside the Closed Polylines that delineate the polygons. When such a map is imported to ArcGIS, the connection between the polygons and their identifiers must be re-established. This can be done with a Spatial Join. The layer with the polygons is the Target Features layer, and the Annotation Features layer that the AutoCAD Text entities have become is the Join Features layer. The Join Operation is JOIN_ONE_TO_ONE. A new layer will be created with the polygons, and its Attribute Table will include the text-string identifiers.

These identifiers can then be the basis of attaching other data tables (in the form, for example, of a spreadsheet) to the polygons. By default, identifiers in an Annotation layer made this way will be text fields. If the external data table has identifiers in a number field, one or the other must be changed, as ArcGIS will not recognize a match between a number field and a text field. To change the field type in ArcGIS, create a new column in the table with the desired field type, and use the Field Calculator to calculate its values based on the existing identifier field. The external table can then be linked to the polygons with the Join Attributes from a Table tool found in the Join Data dialog. The field containing the identifiers must be selected for both the Annotation Table and the external data table. The columns from the external table will be added to the Annotation Table. The entire structure can then be saved as a Shapefile with the Export Data tool.