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TECHNICAL FIELD 
   This invention relates to ground cloths for use between the ground and a ground engagable floor of a tent or similar enclosure for protecting the floor from dirt, abrasive wear and ground moisture. 
   BACKGROUND OF THE INVENTION 
   Ground cloths formed of thin, flexible, light weight, easy to clean, waterproof (nonporous) sheets are known in the art for protecting tent floors from dirt, abrasive wear, and ground moisture. Such ground cloths are commonly sized slightly smaller than a tent floor, or footprint, so that moisture dripping down the sides of a tent will not land on exposed portions of the ground cloth and collect under the tent floor, between the ground cloth and the tent. With tent floors that are not entirely waterproof, such moisture tends to seep into the tent through the tent floor. 
   Ground cloths formed of DUPONT™ TYVEK® have also been proposed. TYVEK® is a high-density polyethylene material that, while not considered to be strictly waterproof, is designed to resist water penetration. Consequently, it does not have the ability to quickly drain away the quantities of water that may accumulate during a rainstorm. 
   Conventional wisdom relative to tent ground cloths is that a ground cloth should be waterproof. However, a ground cloth capable of protecting a tent floor from dirt, abrasion and ground moisture, while also limiting the retention of moisture between the tent floor and the ground cloth, is desired. 
   SUMMARY OF THE INVENTION 
   The present invention provides an improved ground cloth formed of a thin, lightweight, easy to clean, durable sheet, that is adapted for use between the ground and a ground engagable floor of a tent, or the like, to protect the floor from contamination by the ground. The ground cloth includes a plurality of drain holes through the sheet adapted to drain moisture through the sheet to the ground to prevent the retention of moisture between the ground engagable floor and the ground cloth. 
   The number, size, arrangement and spacing of the drain holes may be constant or may vary depending upon the application. For example, the diameters of circular drain holes may vary in a range of from 0.1 mm to 3.0 mm and the spacing between the holes may vary in a range of from 2.5 mm to 50.0 mm so that the drain holes make up between 0.1 percent and 10 percent of the ground cloth surface area. In addition, the ground cloth may be sized having edge dimensions smaller or larger than edge dimensions of an associated ground engagable floor. 
   These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a plan view of a portion of a ground cloth according to the present invention enlarged to show an exemplary pattern of drain holes; 
       FIG. 2  is a plan view of a first embodiment of a portion of a ground cloth having the hole arrangement of  FIG. 1 ; 
       FIG. 3  is a view similar to  FIG. 2  of an alternative embodiment with larger drain holes; 
       FIG. 4  is a view similar to  FIG. 2  of an alternative embodiment with smaller drain holes; 
       FIG. 5  is a view similar to  FIG. 2  of a corner portion of an alternative embodiment having increased drain hole density along outer edges of the ground cloth; 
       FIG. 6  is a plan view illustrating an exemplary ground cloth with a repetitive drain hole pattern similar to  FIGS. 2-4  and having smaller edge dimensions than the footprint of an associated tent; 
       FIG. 7  is a plan view of a ground cloth similar to  FIG. 5  and having smaller edge dimensions than the footprint of an associated tent; and 
       FIG. 8  is a plan view similar to  FIG. 7  but having larger edge dimensions than the footprint of an associated tent. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The term ground cloth refers to a sheet of porous or nonporous material which, may be modified to include drain holes added after or included during manufacture. A ground cloth of this invention is primarily intended for use under a tent floor to protect it from contamination by dirt or moisture on the ground. It could also be used under ground engaging floors that are not part of a tent, such as a sleeping bag, screen house, storage unit or other portable or semi-portable enclosure. 
   The term generally nonporous material is intended to include materials that are waterproof (nonporous) or water resistant but permeable. 
     FIG. 1 , shows a portion of a representative ground cloth  10  of the present invention, shown enlarged to illustrate an overlapping hexagonal pattern  12  of drain holes  14 . The pattern  12  is formed by adjacent triangles  16  having a constant drain hole spacing S between circular drain holes of diameter D. The overlapping hexagonal pattern is an effective one for drainage since, in comparison to a square pattern with the same hole spacing S, a greater percent of the total ground cloth area lies within circles of diameter S centered on the holes. Consequently, for a given hole spacing S, the overlapping hexagonal pattern results in a lower average travel distance for water to reach a drain hole than would a square pattern. Still, other drain hole patterns may be selected if desired and the dimensions D and S may be varied within appropriate limits to be subsequently described. 
   The ground cloth  10  has a sheet thickness, not shown, which may vary to provide suitable strength, flexibility and weight of the sheet material used to form the ground cloth. Any suitable plastic ground cloth material may be utilized, for example, polyethylene sheet having a thickness in a range of 2-10 mils. In exemplary embodiments, the edge dimensions of a rectangular ground cloth  10  range from 2 feet by 7 feet to 10 feet by 18 feet. 
     FIG. 2  shows a first preferred embodiment of ground cloth  10  having the drain hole pattern of  FIG. 1  and wherein the drain holes  14  have nominal diameters D of about 1.0 mm and a spacing S of about 10 mm. 
     FIG. 3  shows a variation of ground cloth  10  having the drain hole pattern of  FIG. 1  and wherein the drain holes  14  have nominal diameters D of about 3.0 mm and a spacing S of about 10 mm. 
     FIG. 4  shows another variation of ground cloth  10  having the drain hole pattern of  FIG. 1  and wherein the drain holes  14  have nominal diameters D of about 0.3 mm and a spacing S of about 10 mm. 
     FIG. 5  shows an alternative embodiment of ground cloth  17  having a center portion  18  with a drain hole pattern similar to  FIG. 1  and an edge portion  20  with a differing drain hole pattern. The drain holes  14  in the center portion  18  have nominal diameters D of about 1.0 mm and a spacing S of about 10 mm. However, the drain holes  14  in the edge portion  20  retain nominal diameters D of about 1.0 mm with a spacing S of less than 10 mm to increase the density of the drain holes at the edge portion. 
   The edge portion  20  preferably has a drain hole area ratio that is 2 to 4 times that of the center portion  18  of the ground cloth. Thus, if the center portion  18  has a drain hole ratio of 1 percent, the edge portion  20  may have a drain hole ratio of from 2 to 4 percent. 
   It should be understood that the diameters D of the drain holes  14  and spacing S between the drain holes as illustrated in  FIGS. 2-5  are merely exemplary. Accordingly, the diameters D of the drain holes  14  preferably lie in a range of from 0.1 mm to 3.0 mm and the spacing S between the drain holes preferably lies in a range of from 2.5 mm to 50 mm. Based on the upper end of the preferred range of hole spacing S, and a uniform square pattern of holes, it also follows that it is preferable that greater Than 75% of the total ground cloth area would lie within circles of 50 mm diameter (25 mm radius) centered on the holes. 
   Preferably, the surface area of the drain holes  14  should constitute between 0.1 and 10 percent of the total surface area of ground cloth  10  or  17 . This ratio provides adequate moisture flow rates (greater than 10 mm of liquid per hour) to prevent the retention of moisture between the floor and the ground cloth while minimizing contact between the floor and the ground to reduce abrasive wear and dirt transfer to the tent floor. 
   In use, a ground cloth  10  or  17  is initially spread over the ground and a tent  22  having a floor  24  is erected over the ground cloth. 
     FIG. 6  illustrates the relationship between a tent floor  22  and a ground cloth  10  having drain holes  14  configured as shown in  FIG. 1 . The tent floor  24  has larger edge dimensions than the ground cloth  10  so that the tent floor slightly overhangs the ground cloth. The edge dimensions of the ground cloth fall within 95-105 percent of the specified dimension of the tent floor has been inserted. 
   As the tent  22  is exposed to moisture in the form of rain or dew, the moisture tends to run down the sides of the tent, not shown, onto the ground surrounding the tent. However, a portion of the moisture draining from the sides of the tent  22 , or flowing along the ground, may seep below the floor  24  and between the ground cloth  10  and the floor. As moisture travels between the floor  24  and the ground cloth  10 , toward low spots in the ground, the moisture is drained through the drain holes  14  to the ground. This reduces moisture retention between the floor  24  and the ground cloth  10  and thereby minimizes moisture transfer through the floor. 
   In addition to the simple drainage of water through the drain holes, the holes enable a vapor transport mechanism to remove either residual moisture that remains after any bulk water has drained out, or ground moisture that may on occasion come up through the holes. The vapor transport mechanism is driven by a tent interior temperature that is greater than the ground temperature, a situation that will usually exist due to heat provided by one or more of the following elements—the sun, the atmosphere or tent occupants. The temperature gradient will generate a vapor pressure gradient across the ground cloth, and thus a flow bias that results in a net transport of moisture through the holes to the ground below. In spite of the presence of holes, a tent is therefore effectively isolated from ground moisture by the present invention. 
     FIG. 7  illustrates the relationship between a portion of a tent floor  24  and a ground cloth  17  having drain holes  14  configured as shown in  FIG. 5 . The tent floor  24  has larger edge dimensions than the ground cloth  17  so that the tent floor slightly overhangs the ground cloth. 
   As moisture first flows over the edge portion  20  of the ground cloth  17 , the edge portion  20 , having a higher drainage surface area, drains a majority of the moisture to the ground to reduce the likelihood of moisture reaching the center portion  18  of the ground cloth. The center portion  18  having less drainage surface area allows any errant moisture that has passed the edge portion  20  to be directed into the ground. The lower drainage surface area of the center portion  18  also minimizes contact between the floor and the ground to reduce ground abrasive wear and dirt transfer to the floor. 
     FIG. 8  illustrates the relationship between a corner portion of a tent floor  24  and a corner portion of a ground cloth  17  having drain holes  14  configured as shown in  FIG. 5 . The tent  22  has smaller edge dimensions than the ground cloth  17 , causing the edge portion  20  of the ground cloth to extend slightly outward beyond the floor of the tent. 
   As the tent  22  is exposed to moisture in the form of rain or dew, the moisture tends to run down the sides of the tent onto the edge portion  20  of the ground cloth  17 . The edge portion  20  having a higher drainage surface area extending beyond the edge of the tent, drains a majority of the moisture to the ground to reduce the likelihood of moisture reaching a center portion  18  of the ground cloth. The center portion  18  having less drainage surface area allows any errant moisture that has passed the edge portion  20  to be directed to the ground. The lower drainage surface area of the center portion  18  also minimizes contact between the floor and the ground to reduce ground abrasive wear and dirt transfer to the floor. 
   If desired, the circular drain holes, previously discussed, may be replaced with various other shapes, such as slits, squares, rectangles, polygons, ovals. 
   Although the invention has been described by reference to certain specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiment, but that it have the full scope defined by the language of the following claims.

Summary:
A ground cloth is adapted for use between the ground and a ground engagable floor to protect the floor from contamination by the ground. The ground cloth includes a plurality of drain holes for draining to the ground moisture entering between the ground engagable floor and the ground cloth to minimize the retention of moisture between the ground cloth and the floor.