The Project GIS
To
create the GIS, I used the ArcMap and ArcCatalog components of ArcGIS 10, for
which my institution has a site license. I have been using GIS on and off since
the late 1980s. Over the past two decades, GIS has gone from typing in commands
to selecting them from pull-down menus, from relatively little functionality to
high functionality, and from static dot-matrix printouts to high quality,
dynamic output on the web. In parallel, the data sources have gone from having
to convert them to digital format yourself, to a limited range becoming available
on floppy disk, to many becoming available on the web in digital form. The
latest innovation, and the one that prompted me to get into a relatively large
GIS project for the first time, has been the addition of a temporal function to
ArcGIS, finally making it quite easy to display changes in spatial patterns
over time. Its productivity has not kept up with its impressive functionality, necessitating
a lot a mouse clicks and keystrokes to accomplish many tasks that could be much
better integrated, and its temporal functions remain limited compared to its
spatial ones. But ArcMap has reached the level of utility that makes it
essential to my research and instructional programs and, in fact, for anyone who
does historical research.
The
first stage of turning the prior work with the databases into a GIS involved
setting it up and importing the data. I will be a bit more step-by-step about
this stage because I assume those interested in the digital humanities have
less experience with ArcGIS than with Excel.
For the
initial setup of the GIS, I created a folder named ACLS_GIS to keep the project
in, started ArcMap, used the File menu to start a New GIS, and saved it in the
folder as ACLS_GIS.mxd. Then I went to the Customize menu and selected ArcMap
Options to make sure that “Make relative paths the default” was selected, which
makes it easier to move the project to another folder or computer. At the same
time, you can check around the other tabs of the ArcMap Options dialog box to set
other options to your personal preferences. Then I used the Windows menu to open
the Catalog window, navigated to the folder ACLS_GIS, right clicked in it,
selected New, selected File Geodatabase (not Personal Geodatabase, which is
more limited), and named the resulting file ACLS_Geodatabase.gdb. That
database, indicated by a special icon, has a specific format used by ArcGIS,
will contain many of the files needed for the GIS, and will keep track of them
and their relationships. The last step of the setup involved going to the File
menu, selecting Add Data, selecting Add Basemap, and selecting one of the base
maps that appear, everything from a basic gray outline to imagery from the Bing
Maps Aerial view. The base map is the underlying map on top of which the data
will be represented, and it can be changed for one of the other base maps at
any time. I selected the Terrain base map, which includes four sub-layers named
Reference, Borders and Places, Shaded Relief, and Terrain. For reference in the
next step, the base maps use the Web Mercator projection of the WGS 1984 geoid,
also known as the World Mercator; its projection reference file is named WGS_1984_Web_Mercator_Auxiliary_Sphere,
and its datum reference is named D_WGS_1984 (more on this in the next step).
The next
step involved adding the data. To add the worksheets in the Excel workbook
named ACLS_Voyages_Data.xlsx to the GIS, I used the Folder Connection button (a
folder icon with + on it) at the top of the Catalog Window to navigate to the
folder that contained the workbook and establish a link to the ACLS_GIS folder
and its ACLS_Geodatabase. I then opened each of the worksheets in turn (for
example, Middelburgs_Welvaren_A$), right clicked it, and selected Create Feature
Class From XY Table to open the dialog box that adds the data to the GIS. In
the dialog box, I selected the longitude column as the X coordinates, the
latitude column as the Y coordinates, and GCS_WGS_1984 as the Geographic Coordinate
System of the Input XYs. Next, under Advanced Geometry Options, I selected Use Map
Spatial Reference, which was the previously mentioned WGS 1984 Web Mercator.
Those selections told the GIS the geographic reference of the data being added
and the way in should project them onto a flat map. Without that information, the
GIS has no idea if all those latitude and longitude numbers in the worksheet are
in degrees, meters, kilometers, or something else; and it has no idea how to
project those positions, which were taken from the curved surface of the Earth,
onto a flat map. Then, still in the dialog box, I navigated to the ACLS_Geodatabase
and saved the resulting Feature Class Files with the name of the vessel (for
example, Middelburgs_Welvaren_A). Opening the ACLS_Geodatabase shows all of
those files with an icon that indicates they are point features. After creating
point feature class files for each of the voyages, I did the same for the
Ports_and_Landmarks worksheet. (Note that if ArcMap does not recognize an Excel
worksheet, saving it in an older version of Excel typically resolves the issue,
in other words, as file.xls instead of file.xlsx.)
The next
step results in all those data actually appearing on top of the base map, a
gratifying moment in any GIS project. I used ArcMap’s Windows menu to open the Table
of Contents window, selected the List by Drawing Order button at the top of
that window (on the far left), went to the open ACLS_Geodatabase in the Catalog
Window, and dragged each of the Feature Class Files to the Table of Contents
under layers, in alphabetical order and on top of the base map. The order of
the layers in the Table of Contents represents the order of the layers on the map,
with all the voyages therefore drawing on top of the base map. All the voyages
then appeared as a series of linear point patterns, the overall pattern emphasizing
the triangular routes of the slave trade in the North Atlantic. Checking or
unchecking on any of the layers in the Table of Contents turns that layer on an
off. Dragging them rearranges their order in the Table of Contents and on the
map. Right clicking any of the layers and selecting Open Attribute Table opens
a table that shows all the cargo, date, latitude, longitude, and other fields
associated with each of the features, or daily positions, in that layer.
Selecting a particular day or range of days in the attribute table, selects its
point symbol on the map and vice versa.
Once all
the data have been imported into the GIS and represented as point symbols, the
next stage involves the use of various tools to alter the data layers and
derive entirely new types of layers from them. Those tools are accessible in
ArcMap by selecting the Geoprocessing menu and then ArcToolbox or by starting
ArcCatalog.
The
first tool I used was the Convert Time Field tool to get the dates of each position
into a standard ArcMap format to use the temporal function. I opened the tool (in
the ArcToolbox under Data Management, then Fields), navigated to and opened the
ACLS_Geodatabase, selected each of the voyage Feature Class Files in turn,
selected their date field, which was in a yyyymmdd format, and converted them
to the ArcMap, standard time format. Opening the attribute table revealed that
the tool has added a new field, DATE, to the attribute table and populated it
with dates in the correct format while leaving the original yyyyddmm date field
intact.
Next, I
used the Points to Line tool to create lines to underlay the points that
defined each voyage, which I believed would better define the route of each
voyage by connecting its daily positions and increase the symbolization, labeling,
and other visualization opportunities. I opened the tool (under Data
Management, then Features), navigated to and opened the ACLS_Geodatabase, selected
each of the voyage point Feature Class Files in turn, selected the field that
contained the voyage name as the Line Field, and saved the resulting line
Feature Class files in the ACLS_Geodatabase, naming them for the voyage with
_line appended (for example, Middelburgs_Welvaren_A_line). I then dragged each
of the line Feature Class Files to the Table of Contents, placing them
immediately underneath their respective point layers. A check of each line
layer’s attribute table reveals that it contains a field with the voyage name,
which can later be used to label the line. Then I selected Start Editing from
the Editor menu and used the split tool to cut each line at the beginning and
end of the segments of the voyage, deleting the superfluous segments: for
example, the segments that cut overland across West Africa from the coast lf
Liberia to the coast of Ghana because of the missing days and positions while
the vessel followed the coast trading for slaves. This changed the attribute
table for each line by adding a record for each new segment. Finally, in the
Table of Contents, I selected both the point and line layers for each voyage,
right clicked, selected Group from the menu, and combined them as sub-layers of
a single layer, naming it after the vessel and voyage (for example, Middelburgs_Welvaren_A).
I then
customized the way in which the GIS represented each voyage by right clicking each
layer and selecting Properties. The dozen tabs of that dialog box allow you to
set everything from how the temporal display works, to the style and color of
the symbols, to the labels, to the scales (zoom levels) at which the layer is
visible. To set the time function for the point layers, for example, I selected
the Time tab, enabled time, selected the DATE field, and set the time interval
at 1 day. To label each point or line segment, I used the Label tab, selected
label features, selected the date field for the points layers and the voyage
name layer for the line layers, and set the scale range at which the labels
would come into and out of view. To connect to TSTD, I used the HTML Popup tab
to select the TSTDURL field to open a search results window for a particular
voyage in that database’s website whenever someone selected one of the point
symbols for that same voyage in the GIS.
The most
time intensive task turned out to be setting the symbols that would represent
each daily position. To do that, I used the Symbology tab in the Properties
dialog box. Since I wanted each daily position to reflect the cargo, I selected
Categories, Unique Values, and the Cargoes field in the attribute table. I then
assigned standard colors to each type of cargo: gray for unknown, brown for
tasajo, and so on. In addition to the standard symbols included in ArcMap,
users can create their own symbols and save groups of symbols under unique
headings so the process becomes somewhat more productive.
By that
point, the GIS had much of the data and functionality I had planned. With all
layers visible, it displayed all the voyage segments as lines overlain with the
daily position points, each with a unique symbol that indicated the cargo. At
broad scales all the voyages blended together and large-scale patterns were
evident, such as the triangular trade between Europe, Africa, the Americas, and
back. Zooming in revealed individual voyages, their lines labels by name and
their points labeled by date. Queries to search for particular attributes (Selection
menu, then Select by Attributes) could be run to select all voyages that
carried, for example, enslaved Africans. And, when time was enabled by
selecting Open Time Slider Window (the clock icon on the main menu bar), and the
GIS run through its temporal extent, the point symbols associated with each day
appeared and disappeared in sequence. Selecting Time Slider Options from the
Time Slider Window menu bar allowed changes to the length of the period
represented, the speed at which it ran through that period, and so on. Such
functions were enhanced by running the Add Attribute Index tool (under Data
Management, then Indexes) for each of the Feature Class Files so that ArcMap
could quickly locate records.
The
final major task to ready the GIS for use involved correcting the positions
added in Excel to represent missing days, latitudes, and/or longitudes as well
as check the integrity of each of the other daily positions for each voyage. I
accomplished that objective by advancing through each voyage, day by day, position
by position, with its attributes table open and editing turned on. Positions
that had been added for missing days with last known latitudes and longitudes,
for example, where selected in the attribute table and dragged to interpolated
positions between known ones. Position makers that appeared too close to shore
were moved off shore on the bases of the attribute table information, which
sometimes named a landmark, its bearing, and its distance. Position markers
that had been given in the logbooks but appeared erroneous relative the
preceding and following days were also corrected at this point, for example,
when a logbook reported a positive latitude as a negative one or vice versa, as
sometime happened near the equator.
Once all
the voyage data were complete and correct as far as the available data allowed,
the hurricane Excel worksheet was converted to a points Feature Class File and
added as a layer in the Table of Contents. I used the ArcToolbox to create a
DATE field, time enabled the layer, and applied symbols to differentiate among
categories of storms. Even reduced to
1851-1900, the attribute table had 9366 records.
By this
point the GIS was highly functional. It made it possible to query intercepts in
time and space between hurricanes and vessels. It revealed the pattern of
deaths and disposals overboard on Dutch slaving voyages, the captains of which
have left us a daily record of such deaths in the logbooks. And therefore the
time had arrived to begin implementing it, or at least parts of it, as a Web
GIS.
Selected References
- Bodenhamer D. J., J. Corrigan,
and T. M. Harris, eds., The Spatial Humanities: GIS and the Future of
Humanities Scholarship (Bloomington: Indiana University Press, 2010).
- Gregory, I. N., and P. S.
Ell, Historical GIS: Technologies, Methodologies
and Scholarship (Cambridge: Cambridge University Press, 2008).
- Hunter, Richard, Methodologies for Reconstructing a
Pastoral Landscape: Land Grants in Sixteenth-Century New Spain, Historical Methods 43 (2010): 1-13
- Knowles, Anne Kelly, ed., Past Time, Past Place: GIS for History
(Redlands, California: ESRI Press, 2002).