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Grass Manual: Notes on Distributional Maps

The grass distribution maps posted at this Web site are generated by ArcView from the geographic database developed by Barkworth and Capels for the Manual of North American Grasses. They are based on a wide range from sources (Barkworth and Capels 2000; current listing).

Data Files
Each map draws on at least two, and possibly four, data files. One file contains county level data for the contiguous 48 states of the U.S.A. The color of the shading indicates the source of the record.

The other three files are all locality files, meaning that they contain latitude and longitude data, plus information about the data source. One of these contains data from diverse sources, ranging from specimens to publications of various kinds. The second locality file is generated from the Intermountain Herbarium database; the third locality file is similar in structure to the Intermountain Herbarium database but contains information from other herbaria or from individual collectors.

We have only just begun to develop the "Other Herbaria" file, so it does not contain many records. Our ultimate goal is to have the maps drawn by calling on information in the databases of individual herbaria.

At present, the maps are prepared by generating jpg images and posting these images to the web. We are working on making them available dynamically and providing active links from the locality dots to the files from which they are generated so that anyone interested can find out the basis of the record.

Sources of Data
Some contributors provided us with county level information. Such records appear in red. Others provided shaded area maps which we could not easily enter into the database so it appears as if we have no data from the contributor, but this is not necessarily the case. If we were given locality data, the data will show as a blue dot on the maps and the information provided is in the database.

Some contributors were not able to provide us with distribution information because they are not employed by institutions or companies that can provide the resources needed for such work. It should be noted that contributors received no support for preparing their treatments from the project. Some have positions that encourage such work; many do not. Without the generosity of the contributors, projects such as the Manual would be impossible.

Other sources of data included atlases, checklists, and a variety of other publications; databases from herbaria or, in the case of weeds, agencies within the USDA; and from a variety of individuals who were so kind as to send us their data.

The areas shown in color correspond to counties. Most herbarium specimens from the contiguous United States note the county in which a specimen was collected, and counties are frequently the unit of record for state distribution maps and for checklists. This makes it relatively easy to acquire county level data. When a county has been shaded it merely means that the species concerned has been found at least once in the county, not that the species occurs throughout the county.

The colors indicate the class of data on which a county record is based.

Cautionary Notes
The only verifiable distribution maps are those based on herbarium specimens. Ideally, all the maps in the Manual would be based on specimens that had been identified by the contributors. For most genera, that is impossibly idealistic. There are simply too many specimens in too many herbaria for any individual to check, particularly individuals that are receiving no compensation and no staff support for their work, the condition of many contributors.

A second best approach is to ask taxonomists in various parts of the country to check the specimens in their region, using the treatment developed by the contributor. This too is an enormous task, and one that has not been considered in the Manual's funding to date. We are soliciting funding to fill in some of the regions for which we have few data but we cannot, realistically, expect to obtain the funding needed to complete all the maps in this manner.

Our database takes an approach that many would not consider acceptable: we enter published data from a variety of sources, often not knowing who has done the identification nor what treatment they followed. If the Manual contributor adopts a narrower interpretation of a species than the person whose work we used in compiling the database, our maps will be seriously in error. Fortunately, the number of times that this has happened is relatively few - but it is a problem with some species.

We shall send all maps to the contributor before publication, even publication on the Web, but this may not resolve all the problems. In many respects, preparation of detailed maps should be delayed until after treatments have been published. We prefer to provide maps and encourage those who find them in error, or questionable, to draw attention to the specific problems. We shall create a message center where such questions can be posted and others, as well as the contributors and ourselves, can respond. Since "ourselves" currently consists only of Kathleen Capels and myself, it is unlikely that we shall be able to follow up on any but the major problems.

The very best solution would be to have a Web site that would provide dynamic distribution maps in which each locality record is linked to a herbarium record. Technologically speaking, it is very doable - but no one wants to pay for the unglamorous and time consuming task of placing herbarium data into a database. Many herbaria have begun to do so, so a pilot scheme in which their data are made available is feasible. Once available, anyone who needed to be sure that a particular distribution record was correct would be able to determine where the documenting specimen was located and inquire about having it verified, in return, of course, for appropriate compensation for the work involved.



Picture of Achnatherum. Photographer unknown.