Scholars originally used GIS for organizing data and
creating maps for presentations. These uses are still common, but
GIS software can now perform more analytical functions. The next
two sections describe some of the descriptive (basic) and analytical
(advanced) functions of GIS.
A. Descriptive GIS
Managing Data:
Managing the data for large projects is often difficult, and geographical
information is especially cumbersome. GIS uses a framework for
storing and accessing many forms of data, from tabular databases
to images
and maps. The software connects to these files and represents them
in a traditional directory structure. Once the files have been
imported to the project, they may be linked to spatial data and
to each other.
For instance, a researcher can create a historically accurate map
of a county, link a census database to another database from a
business directory, and then display various attributes of those
people and
businesses on the county map.
Creating and Overlaying Maps:
The end product of a GIS project is one or a series of maps – which
are simply visual representations of data arranged in a spatial
manner. These maps may display points of reference, analysis of
phenomena, or elements of both. GIS facilitates basic cartographic
tasks, such as creating legends, north arrows, and distance scales.
It also performs the difficult job of accurately projecting a map
to the shape of the Earth’s surface. A GIS also combines any number
of layers and allows their transparency and sequence to be adjusted.
Researchers can therefore combine and compare maps from different
periods and maps that highlight different geographic features.
As well, they can overlay representations of their own historical
analysis on historical or modern maps.
B. Analytical GIS
Below are some of the principal ways scholars have used GIS to analyze
spatial relationships in historical data.
Joining Data:
Creating thematic maps, or maps that display quantitative attributes,
often requires joining data. Linking data to a map requires that
the database(s) have a geographic attribute in common with the
map, be it a coordinate, a town name, a lot number, or a street
or concession address [see the Case
Study].
Censuses and business directories are well suited to this linkage
because they are ordered according to geographic location. Linking
two databases in a GIS requires that they have a common field.
Once this field has been standardized the software can form a join
based on the variable, and the data now found in one table may
be linked to another, and, ultimately, a spatial representation
of the information. For instance, a business account book that
recorded sales of goods to customers in different towns may be
compared to the demographic features of those towns and the distances
between each town and the business. Or if the business recorded
transactions with rural residents, and
the accounts can be linked to census data by the client’s name
or lot number, GIS offers a variety of analytical tools for understanding
these rural relationships.
GIS allows integrating and relating data from a variety
of sources. In the discipline of history, there are a wealth of paper
maps and statistical information buried in cabinets, atlases, and
census records that can be digitized, analyzed, and displayed in
a GIS.
Georeferencing:
One problem with using today’s spatial information for historical
study is that landscapes change over time and human geography changes
even more quickly. Place names, populations, and land use change
frequently, but topographical characteristics such as forest growth
and soil quality also evolve. Georeferencing is a tool in GIS that
allows researchers to overlay maps made from modern spatial data
with more historically accurate maps, surveys, and air photographs
(photographed for many places in the early twentieth century).
One form of georeferencing is known as “rubber sheeting,”
where researchers identify several equivalent points, such as boundaries
or intersections, on the modern and historical maps. The GIS software
then reorients and projects the historical documents so they fit
modern coordinates. Then, researchers may reliably compare data at
any given point, or even display the results of their analysis overlaid
on historical maps and images.
Spatial analysis:
Studying historical data is often facilitated by projecting those
data over space. But, sometimes the geographical information about
a place or its environment is one of the only ways to make sense
of certain data. For example, the production of certain commodities
is often based heavily on the producer’s proximity to natural resources
or modes of transportation. With GIS, researchers can more easily
measure the distance between points and analyze the ratios of production
to distance.
The software takes many other environmental factors
into consideration. For instance, elevation data can recreate the
“viewshed” of a particular area, or the points visible to an observer
at any given location. This has helped archaeologists understand
the placement of human objects and landscaping and predict the location
of other archaeological sites. Others use spatial analysis to find
correlation between settlement or other human activity and geographic
features such as altitude or drainage.
Geostatistical analysis, or Interpolation:
GIS can use geographical attributes to help “fill in the blanks”
when information is available for some places but must be interpolated
for others. This tool uses correlation and regression analysis
and considers environmental variables when predicting the most
likely characteristics of any unknown point in a spatial grid.
Geostatistical analysis facilitates the modeling of regional economic
development based on environmental factors and the distance between
points. It has helped historians understand the effect of drainage
on disease, and spatial interpolation has also been used to study
migration where changing census boundaries have frustrated other
quantitative historians. Researchers who use census samples may
also experiment with interpolating the new data they retrieve from
cross-tabulations of manuscript census databases.
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