DIGITAL COVERAGES/SHAPEFILES OF MINE PITS IN THE MINNESOTA RIVER VALLEY

Sara Gran

Minnesota Geological Survey

1997

Introduction

This project is a data set of mine pit locations in the Minnesota River Valley. The project area includes all townships that lie at least partially within a six-mile buffer zone around the Minnesota River. Mine pits were located using a variety of sources, and digitized on top of Digital Orthophoto Quads (DOQs) and Digital Raster Graphics (DRGs) of USGS 7.5 minute topographic quadrangles. Each mine pit is connected with a data table listing its attributes. This report includes specific information on how this data set was compiled, an explanation of the attribute tables, and descriptions of some of the problematic aspects of the compilation of these data.

A data set of this kind has several practical applications. It can be used to calculate the area of resource mining activity in any specified area within the study area. It can also be used to study the distribution of resource availability within the study area and be used to help locate new mining sites.

Methodology

Using several information sources, mine pits (including quarries, gravel pits, sand pits and kaolin mines) were located and digitized as shapefiles (polygons and points) on top of DOQs (digital air photos) and DRGs of topographic maps in the GIS program ArcView 3.0. The mines are represented in three different theme coverages: "gravel," which means pits of unconsolidated material including sand, gravel, glacial till, and in some cases, kaolin; "allpits," which confusingly stands for rock quarries of various types (crushed stone, dimension stone); and "smallpits," mines of any commodity that are less than one acre in surficial area. Each individual mine is referenced in a data table where additional information on the mine is included. The data table includes the following information: ID, resource (commodity mined), activity (active or inactive), county, area of the mine in square meters, area of the mine in acres, MNDOT database ID number for the same mine, and the information source.

· There were two different "base themes" used to make this database: DOQs and DRGs (from UTM Zones 15 and 14). Base themes are consistent throughout each particular county. Blue Earth, Brown, Carver, Dakota, Hennepin, Le Sueur, Nicollet, Redwood, Renville, Scott, Sibley and Yellow Medicine counties were done using DOQs. Big Stone, Chippewa, Lac Qui Parle and Swift counties were done using DRGs. Which zone the latter counties were done in depended on which was available. DRGs in UTM Zone 15 (the same UTM coordinates the DOQs are in) were used whenever possible, at the expense of consistency throughout a particular county.

· As mentioned above, the DRGs were in two different coordinate systems. One coordinate system was the same as the DOQs (UTM Zone 15), so the polygons and points for these DRGs could be made in the same themes and included in the same project as those drawn using the DOQs. Some of the other DRGs, however, were in UTM Zone 14, and the mine polygons and points are therefore made in a completely separate project with separate themes from the Zone 15 ones. The Zone 14 polygons can later be re-projected into Zone 15 so they will overlap properly.

· Polygon formation. Individual polygons and points were drawn with the on-screen digitizing tool. The mine location from any of the sources was found on the DOQ or DRG and a judgment was made to determine if the pit was larger than one acre in area. If so, the digitizing tool was used to make a polygon of the outline of the mine. If not, the mine was marked with a point. There were difficulties in determining the exact extent of some of the mines; in many cases there was little or no distinction on the DOQ or air photo between spoil piles and actual mining area. The polygons therefore include all disturbed land associated with a mine, which may or may not include spoil piles on top of unmined land. For the counties that only had the DRGs (scanned-in 7.5 minute topographic quadrangles), mines were first located on the paper topographic maps, then found on real air-photos (not DOQs) from the NAPP 1991-1992 1:40,000 scale color-infrared set. Then, to the best of the compiler's ability, the mine outlines were freehand-drawn on top of the DRGs. These polygons are therefore less accurately drawn, since they were not drawn directly on top of a DOQ.

Project/theme/attribute file names

The name of the final projects are "XXX.apr" and "XXY.apr." These two projects are the maps of mine pits for the two UTM zones (14 and 15). These projects include only the mine pits and, in the case of Zone 15, county boundaries. In order to look at the DOQs or DRGs along with this project, one will need the required CDROMs.

The following is a chart of shapefile names for the different themes in each UTM zone:

Quarries >1 acre Gravel pits > 1 acre Any mine < 1 acre

Zone 14 quar.shp grv.shp smpits.shp

Zone 15 allpits.shp gravel.shp smallpits.shp

Each theme has a shape file (.shp), a data table file (.dbf) and some other file called ".shx." One needs all of these files on the computer for the theme to open in ArcView 3.0.

Attribute Tables

· "ID" a unique, 6-digit identification number for each mine. ID numbers start with the general-use two-digit county code. The third through sixth digits are numbered sequentially in order of location starting with "1001." Three counties in this study area that have county codes less than 10: Big Stone, Blue Earth and Brown. These are represented as 5-digit numbers due to constraints imposed by the software. For example, the first mine located for this project in Blue Earth county is "71001," rather than "071001." When opening the data table for a particular theme, one will notice that the ID numbers are often not in sequential order. This can be remedied by highlighting the "ID" field header (click on it) then clicking the "Sort Ascending Order" button on the toolbar. This will temporarily reorder the ID numbers so they are in sequential order, making them easier to work with. The reason why the numbers are out of order is because several information sources were used to find the mine locations over the course of compilation.

· "Resource" This indicates what the commodity being mined is (or was). For the hard rock resources, this commodity type was found in the DNR Pit and Quarry Report, and indicates the rock formation (example, Oneota dolomite, Morton Gneiss). It does not differentiate between crushed and dimension stone. That information may be found by looking up its reference in the DNR Report 282 (the Pit and Quarry Report). In the case of the gravel pits, the resource designation is generalized to (in most cases) just "gravel," due to the lack of information on each pit. In many cases there may be a variety of resources mined in these pits, including sand, gravel, cobbles, boulders, glacial till, and glacial lake clays. For the small pits represented as points, again the unconsolidated materials are listed generally as "gravel," and the rock types in the small quarries were found in the Pit and Quarry Report.

· "Activity" For quarries, the activity was determined by the designation found in the Pit and Quarry Report. These data were compiled in 1990, so it is possible that the activity level has changed in some of the mines. Also, in several cases the quarry has several different sections, only one of which is actively being mined. In this situation, the whole quarry is designated as "active." For the gravel pits, this designation is much more ambiguous. This data set was, for the most part, NOT field-checked, so the activity designation was largely based on the appearance of the gravel pit on the DOQ. For example, in some of the large, commercial gravel pits, sand piles and large mining equipment is visible on the DOQ, so it is very likely that that particular pit is active. Other indicators of recent mining activity are the presence of sharp pit edges, lack of vegetation or trees, or vehicles in the pit. Also, for some counties (Yellow Medicine, Laq Qui Parle, Big Stone, Chippewa, and parts of Renville and Redwood counties) there was a field-check done for a Minnesota Geological Survey project (UMRB, Carrie Patterson) in the fall of 1996. In this field area, gravel pit activity noted in the data table is highly accurate. Still other gravel pit activities are known because they have been seen recently by the compiler of this data set. In many cases, however, it is extremely difficult to determine whether a gravel pit is active or not. The activities marked by an asterisk (*) are "best guesses." Unmarked activity designations are to be presumed correct.

· "County" This is the name of the county the mine is located within. All counties are in Minnesota.

· "Area, square meters" ArcView calculates the area for each polygon. Since the DOQ and DRG data uses meters as their map unit, the inital area is calculated in square meters. This was done by creating a data table field named "area, sq. m" then using the calculator function on the toolbar ([shape].returnarea) to automatically calculate the area of each polygon.

· "Area, acres" The area in square meters is then converted into acreage by using the following square meters to acres conversion:

[area, acres] = [area, square meters] * (0.0002471)

(there are 0.0002471 acres per square meter)

· "MNDOT" The Minnesota Department of Transportation also has an ArcView 3.0 database of mine pit locations in Minnesota, mostly just gravel and sand pits. The MNDOT database was used to help locate mines for this project. If a mine in this DNR project corresponds with any mine on the MNDOT ArcView database of gravel pit locations (whether or not the mine was located using that source) the "uniquexx" number from the MNDOT database is put in this column. In some cases, there are more than one MNDOT points that overlap a mine site. When this is the case, one of the MNDOT uniquexx numbers is chosen arbitrarily.

· "Reference" This is the information source used to find each particular mine. The most common reference source is the 7.5 minute USGS topographic quadrangle. The pits and quarries found from the 7.5 minute quadrangles were located by looking for the pit symbols, then finding those locations on the DOQs. In some cases, the mine is not visible on the DOQ, but marked as (usually) a point anyway. Other sources of information include the DNR Pit and Quarry Report, MNDOT county highway maps, soil survey maps (Ramsey County only), Minnesota Geological Survey (MGS) County Atlases (Hennepin, Dakota, and Scott counties only), the MNDOT gravel pit ArcView project, the Minnesota Geological Survey's Upper Minnesota River Basin (UMRB) project field notes, NAPP color-infrared air photos from 1991-2, and DOQs. All of these sources are cited in the bibliography.

Other notes

· In the case of extensive quarrying in a localized area (notable example, the two areas of quarrying near Morton, MN), a large polygon was drawn covering the large outcrop area. This was done because it was impossible to determine the exact locations of about a dozen small quarries.

· There were many inactive mines listed in the Pit and Quarry Report that were impossible to locate either on the DOQs or topographic quadrangles. These are not included in this database. The same thing happened with the MNDOT ArcView project database; there were many pits in that database that could not be found on the DOQs or in other sources. These too were not included.

· The soil survey map was only used to find mine locations (mostly gravel pits) in Ramsey County. This was because Ramsey County is small and had very few gravel pits marked on the 7.5 minute quadrangles. To do this for all the counties would have been very time-consuming.

· Many mine pits mentioned in the source material are now in developed areas. Where it is obvious there is now a building, parking lot or golf course on top of an old gravel pit or quarry, the pit is not included in this project.