Making wise land-management decisions depends on a well-planned and well-executed forest inventory. In this lesson, we will discuss how to use aerial photographs and maps and determine sample plot locations.

Introduction

The first step in planning any forest stand inventory is to locate your property boundaries and mark them. This step will save you a lot of time, and trouble, whether you are setting up a timber sale or developing a management plan.

Maps and Photographs

Maps and aerial photographs are invaluable tools; they enable landowners to observe many features of the area before stepping foot on the site. Many different types of maps are available—tax maps, highway maps, topographic maps, soil survey maps, and others. These maps, along with aerial photographs, locate property boundaries, roads, and special landscape features, and supply information about soils, topography, and vegetation. They indicate areas of special concern, such as streams and wetlands, and are useful for laying out trails and roads.

Topographic Maps

A map is a graphic representation of the Earth's surface, drawn to scale, with both man-made and natural features depicted by symbols, lines, and colors. A planimetric map, such as a highway map, shows only the horizontal (flat) position of constructed and natural features. A topographic map shows both horizontal and vertical positions (features in relief). Topographic maps depict roads, buildings, and other man-made features; forests, wetlands, and bodies of water; plus many other site characteristics.

The U.S. Geological Survey produces topographic quadrangles, or maps, of all U.S. land areas. These maps are available in a number of different scales. The scale expresses the ratio of the distance on the map to the corresponding distance on the ground and is written as a fraction or ratio. Rulers printed on maps also show scale.

How to Use the Scale

The numerator (always expressed as one) represents the map distance; the denominator represents the ground distance:
1:24,000, or 1/24,000, translates to 1 inch = 2,000 feet (24,000 inches), which is useful for viewing small areas;
1:62,500 or 1/62,500 (1 inch = 1 mile); 1:250,000 or 1/250,000 (1 inch = 4 miles) is useful for viewing large areas.

Map symbols depicting natural and constructed features usually appear in different colors, each color representing a class of features. Following are the colors printed on topographic maps and what the colors represent:

Colors represented on a topographic map

Black
Cultural and man-made features

Blue
Water

Green
Forested vegetation (includes vineyards and orchards, etc.)

Brown
Shape and elevation of topography

Red
Boundary lines, large highways, special features

Other colors
Special information indicated in the legend

**Colors represented on a topographic map**
Black	Cultural and man-made features
Blue	Water
Green	Forested vegetation (includes vineyards and orchards, etc.)
Brown	Shape and elevation of topography
Red	Boundary lines, large highways, special features
Other colors	Special information indicated in the legend

Elevation is the height of land surfaces in feet above sea level. Relief, the variation in elevation of the Earth's surface, occurs in the form of hills, valleys, and ridges. The most common method for representing elevation and relief is contouring. Contour lines depict the vertical distance above or below sea level. Starting at the lowest elevation, every fifth contour line—the index contour—is heavier and darker than intermediate contours.

Spacing of contour lines signifies degree of slope. Lines wide apart and evenly spaced indicate a uniform gentle slope. Lines close together indicate a steep slope; the closer together the contour lines are, the steeper the slope.

Figure15-1: Contour lines on a map

Figure 15-1: Contour Lines on a Map

Figure15-2: Spacing between contour lines represents degree of slope

Figure 15-2: Spacing Between Contour Lines

Topographic maps can also be found for Maryland properties in Maryland Department of Natural Resources MERLIN. This is an interactive map where you can choose from a number of layers, one of which is USGS Topographic Quads. To access MERLIN go to https://dnr.maryland.gov/Pages/Merlin.aspx

Soil Survey Maps

One of the responsibilities of the U.S. Department of Agriculture (USDA) Natural Resource Conservation Service (NRCS) is mapping the different types of soil. Soil scientists extract soil samples across a geographic area and make comparisons to known soil types. In addition to observing the steepness, length, and shape of slopes, scientists record the kinds of existing rocks and vegetation. They also conduct laboratory and engineering tests, establish field crop trials, and calculate site indices for major forest tree species. They use this information to name and classify soils and construct soil survey maps.

Soil survey books containing these maps are available on a county by county basis from your local NRCS office. Along with soil descriptions, the books also include past and present land use, depth to the water table, and local climate data. Information in the soil survey book can be applied in managing watersheds and woodlands, in planning engineering works and community development, and for determining the suitability of land for farming, industry, wildlife, and recreation. You should already have a copy of a soil survey from completing an assignment in Lesson 9. If you do not, obtain a copy and determine your soil types.

Other Maps

The tax parcel map, available from the county courthouse, helps in establishing property boundaries. National wetland inventory maps, based on topographic quadrants, locate the headwaters of streams and the presence of wetlands. These maps are on file at your local office of the USDA Natural Resources Conservation Service and the U.S. Fish and Wildlife Service. Natural Heritage Inventory maps locate the position of rare, threatened, and endangered species. Obtain these maps from Maryland’s Department of Natural Resources Wildlife and Heritage Service.

Aerial Photographs

Taken from aircraft, aerial photographs of the Earth’s surface furnish information about topography, vegetation type and density, damage from fires and pests, soil types, and cultural features (roads, buildings, etc.). Photographs are taken along a series of parallel flight lines, spaced to create overlapping shots. It is possible to observe three-dimensional images by viewing pairs of these photographs through a stereoscope.

The agency responsible for the photograph is recorded on each copy, along with the following: county code, flight line number, scale, marks for locating the optical center, and the photograph number, date, and time. Aerial photographs are available in black and white, color, and infrared and in a number of different scales. For the most up-to-date copies of infrared and black and white aerial photos, visit the office of your local U.S. Dept. of Agriculture Farm Service Agency.

Another way to view your property is through satellite images. These can be found by using Google Earth Pro. This is a free download. Here you can view 3D images, compute distances and areas, and print screen images. Maryland Department of Natural Resources as a GIS and map data program with similar features as Google Earth Pro. This program, called MERLIN, can be found at https://dnr.maryland.gov/Pages/Merlin.aspx. This program covers Maryland only and offers layers such as wetlands, critical areas, the Chesapeake Watershed and more.

Determining Acreage

There are several methods of estimating acreage from maps and aerial photographs using planimeters and grids. A planimeter is a mechanical device that reads the size of an area as it traces the boundaries. Transparent grids consist of intersecting lines or dots that form squares; each square is assigned an area value according to the scale of the map or photograph. To gauge acreage, lay the grid over the map or photograph and count the number of squares covering the area. (See Appendix 31, "Using Aerial Photographs—A Layman's Guide.")

As mentioned previously, Google Earth Pro and Maryland Department of Natural Resources' MERLIN can both measure distances and areas. In Google Earth Pro, search for your location. Using the ruler located in the top center of the map, choose "Polygon" for acreage and the measurements for the perimeter and area. Draw your polygon. In MERLIN, click on the "Interactive Map" tab. Under "Operational layers" click on "High Resolution Imagery". Locate your property and then open the ruler at the lower left of the map. Draw your polygon.

Creating a Property Map

To create a working map of the property, start with the information available from maps and photographs. Generally, the best way to draw this map is to trace directly from the aerial photograph. Be sure to include buildings, roads, streams, ponds, fields, woodlands, wetlands, hedgerows, pastures, idle fields, and any special features of the property. Subdivide forested areas according to shared features, such as areas with similar vegetation or areas with vegetation of similar age. Designate each of these areas as a separate stand or management unit, and maintain a separate inventory for each stand. Tailor the management of each stand to its specific needs.

Figure15-4: Typical features of a forest stand map.

Figure 15-4: Forest Stand Map

Information for Ordering Maps and Aerial Photographs

Contact your local Natural Resources Conservation Service, Farm Service Agency, or Forest Service office to view aerial photographs, topographic maps, and soil survey maps (See Appendix 8 for office locations and phone numbers.) For purchasing maps, contact:

Information for Ordering Maps and Aerial Photographs
Aerial photographs:	U.S. Dept. of Agriculture Farm Service Agency Aerial Photography Field Office 125 S State Street Suite 6416 Salt Lake City, UT 84138 801-844-2922 https://www.fsa.usda.gov/programs-and-services/aerial-photography/index
Topographic maps:	U.S. Geological Survey Information Services 12201 Sunrise Valley Drive Reston, VA 20192 1-888-ASK-USGS (1-888-275-8747) https://www.usgs.gov/products/maps
Soil survey map:	Your local U.S. Dept. of Agriculture Natural Resources Conservation Service Office or state or local Soil and Water Conservation District office. http://www.nrcs.usda.gov

Inventory Design

After completing a property map, the next step in the inventory or cruise is determining what types of field information to collect, how many field plots are necessary as a sampling, and where the plots will be located. A complete inventory of a forest of many acres containing thousands of plants is rarely necessary. Rather, paint a picture of the larger forest by measuring small sample areas, making certain the samples truly represent the entire forest. Your local forester can assist you in your planning.

New Technologies in Forest Inventory

Recent advances in computers and space technology have resulted in these new systems for keeping track of forest ecosystems—remote sensing by satellite, global positioning systems (GPS), and Geographic Information Systems (GIS).

Scanned images from satellites offer up-to-the-minute information on large land areas. Images are recorded as electronic data when satellites pass above the Earth’s surface. Computers reconstruct the data into characteristics in relationship to each ground point. What satellite imagery loses in precise detail—the kind available from standard aerial photographs—it makes up in accuracy and speed.

Global positioning systems are based on radio signals broadcast to Earth by satellite and intercepted by receivers on the ground. The receivers, some as small as a wristwatch, are carried into the field, mounted on vehicles, or incorporated into data recording devices. There are also GPS smartphone apps. They allow the operators to record their ground positions anywhere on Earth, to within 6 to 30 feet of accuracy. Among the many uses for GPS are fast and precise mapping of roads, property boundaries, field plots, and locations for fire control.

The computerized Geographic Information Systems combine information from many different types of maps, such as maps for soil, climate, wildlife, plants, and social data. GIS makes it relatively easy to track ecological conditions, habitat components, land-use patterns, forest health, and other variables, over large areas for long periods of time.

Plot Locations

There are two ways to locate field plots in the stand: randomly or systematically. Generally speaking, scattering plots on a random basis, rather than on a systematic one, requires spending more time and effort in the field searching for plots. Most forest inventories employ a systematic approach, using cruise lines, or survey lines, spaced apart at given distances, with plot points spaced at given intervals along the lines.

Figure15-5: Random sampling versus systematic sampling

Figure 15-5: Random Sampling vs. Systematic Sampling

Type of Plots

There are three types of field plots: strip plots, fixed area plots, and variable area plots.

The strip plot, also known as a strip cruise, is commonly used for estimating timber volumes. During the cruise, all merchantable trees are measured, within a given distance—usually 1/2, 1, or 2 chains—on either side of a cruise line.

Figure 15-6tain. — **Figure 15-6: Strip Sampling.**

In fixed area plots measurements are taken in a small area, usually 1/10 to 1/4 acre in size. The correct plot size depends on the number of trees per acre, the size of the trees, and the purpose of the survey. Generally speaking, the plots should contain an average of at least 8 to 10 trees. Although the area can be any shape, square and circular plots are most common. Fixed area plots are used for all types of forest inventories, from estimating timber volumes to conducting ecological surveys.

Variable area plot measurements have the same uses as fixed area plot measurements. Measuring a variable area plot generally requires only one person, whereas the other two methods require two or more for collecting the data. In addition, it takes less time to conduct a variable area plot survey than to establish fixed area plots, and variable area plot surveys are a more accurate measure of timber volume on the stand. Variable area plots (also known as prism or point sample plots) measure all the trees with a diameter at breast height that exceed a certain angle when viewed through a wedge prism. The prism itself is a thin wedge of glass that bends light rays as they pass through the glass.

To create a variable area plot, first establish a plot center. Hold the prism over this plot center, at breast height, and then walk in a circle around the prism. Note that the section of the tree sighted through the prism at DBH appears to move off center. If that section of the tree remains within the vertical lines of the actual tree above and below the prism—in-tree—measure and tally the tree accordingly. If that section appears totally outside of the lines—out-tree—do not measure or tally the tree. In cases where an edge of that section is partially in line with the original tree—a borderline tree—do not measure every tree, but every second tree.

Figure 15-7, Using a Wedge Prism, shows how trees at three different angles will appear through the prism. Once you master using a prism, it offers a fast, accurate, and simple way to measure and tally trees in a cruise.

Figure 15-7: techniques for using a wedge prism

Figure 15-7: Using a Wedge Prism. Image courtesy University of Houston-Clear Lake.

Number of Plots

How many plots need to be sampled depends on the purpose of the survey and the composition of the stand in terms of size, age, species, etc. The more area sampled, the more accurate the inventory. A landowner selling highly valuable trees might want to inventory one hundred percent of the area. In most cases, however, a smaller sample is adequate.

A practical method for determining number of plots is to base calculations on a percentage of the whole: multiply the percentage of land area you want to sample by the total acreage; then, divide by the size of the plot.

Percentage of survey x total acreage ÷ plot size = Number of sample plots

For example, if you want to survey 10 percent of a 75-acre forest, and estimate that a 1/5-acre plot is necessary, the number of plots is 0.10 x 75 = 7.5 ÷ 1/5 = 38.

Table 15-1, Commonly Used Sample Plot Sizes, shows sample plot sizes for circular and square plots. Your local forester can help you determine what percentage of area to survey for an accurate cruise.

**Table 15-1: Commonly Used Sample Plot Sizes**
Use	Size (in acres)	Circular plot radius (in feet)	Square plot side dimension (in feet)
Regeneration	1/1000	3.7	6.6
Mature trees	1/10	37.2	66.0
	1/5	52.7	93.3
	1/4	58.9	104.4

A Note on Notes

Field notes are permanent records, consisting of explanatory notes, sketches, and numeric data. The importance of taking good field notes—written clearly so anyone can read and interpret them—cannot be overemphasized. Your sample may represent thousands of trees, and it is easy to forget information about them unless it is written down. In addition, the more accurate your notes and cruise data are, the easier it will be to calculate volume and make property maps.

An easy way of counting trees in a plot is the dot tally system. This system substitutes dots and lines for numbers in a way that easily allows you to add to a tally without having to erase or cross out anything.

Figure15-8 - system of dots and lines used by foresters called the "dot talley system"

Figure 15-8: Forester's Dot Tally System

A portable data collector, which stores data electronically, is an alternative to paper and pen. Information from these systems is transferred directly into a computer for analysis, making them useful for tallying a lot of numeric data. Their disadvantage is in restricting your ability to make extensive hand notes and diagrams. Because of the expense of portable data collectors, most county foresters do not use them.

Keeping a Tally

Figure 15-9: A Sample Tally Sheet. — **Figure 15-9: Sample Tally Sheet.**

As you cruise your woodland measuring trees, use tally sheets: keeping count requires a systematic approach, and tally sheets are easy to make and duplicate, to fit your needs. A tally sheet such as the one depicted in Figure 15-9, Sample Tally Sheet, is a simple yet flexible method for keeping track of large amounts of field data. It enables you to list the species to be tallied any way you like. In a mixed pine and hardwood forest, the listing might be species by species as in Figure 15-9; in a young southern pine forest being measured for a pulpwood sale (except for large pines of sawtimber size), the column headings would only list pine.

If you use a tally sheet similar to the one in Figure 15-9, you will need more than one sheet if you are tallying more than four species or groups of species. You can customize tally sheets to fit your needs.

To keep from counting a tree twice during a cruise—especially easy to do during a total cruise—either mark the tallied trees with chalk or lumber crayon, or mark the first tree in a circular plot, so you will know when you have completed the 360 degree sweep of the plot. Always mark on the same side (e.g., south) so you know quickly where to look for these reminders.

Since you are making an effort to estimate as closely as possible the actual amount of usable wood to be sold, do not tally portions of trees that are obviously rotten, severely crooked, or otherwise unusable. Expect buyers to deduct an additional amount for internal decay or other hidden defects when they scale the sawlogs at roadside or the mill.

References

Goff, G.R., J.P. Lassoie, and K.M. Layer. 1985. Timber Management for Small Woodlands. Cornell Cooperative Extension. Information Bulletin 180.
Hamilton, R.A., and M.A. Megalos. 1995. Using Aerial Photographs - A Layman’s Guide. North Carolina Cooperative Extension Service.
Kjellstrom, Bjorn. 1967. Be Expert with Map and Compass. New York: Charles Scribner Sons.
Marquis, D.A., R.L. Ernst, and S.L. Stout. 1984. Prescribing Silvicultural Treatments in Hardwood Stands of the Alleghenies. U.S. Dept. of Agriculture Forest Service. General Technical Report NE-96.
Oregon State University Extension Service. 1989. Forest Measurements. Stand Volume and Growth: Getting the Numbers. Extension Circular 1190.
Sharpe, G.W., C.W. Hendee, W.F. Sharpe, and J.C. Hendee. 1995. Introduction to Forest and Renewable Resources. New York: McGraw-Hill Inc.
U.S. Dept of Agriculture Soil Conservation Service. 1973. Soil Survey of Cecil County, Maryland.

Assignments

Read:

Appendix 29, “How to Determine Your Property Boundaries.”

Fazio, James R. The Woodland Steward: A Practical Guide to the Management of Small Private Forests. Pages 21–22, 38, and 199–200.