Topic 4: Toward an Honest GIS
Spatial Reasoning book
The This, That, There Rule — describes creating a “Shadow Map of Certainty” that characterizes the spatial distribution of probable error
Spawning Uncertainty — identifies a procedure for tracking error propagation in map overlay
Avoid Dis-Information — describes the calculation of a localized Coefficient of Variance map
Empirical Verification Assesses Mapping Performance — describes procedures for assessing mapping performance through Error Matrix (discrete) and Residual Analysis (continuous)
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The This, That, There Rule
(GIS World, July 1994)
You have heard it before, “This map says we are on the mountain over there.” Yep, maps are not always perfect regarding precise placement of discernable features. Monmonier, in his insightful book How to Lie with Maps (1991), notes that it’s “not only easy to lie with maps, it’s essential …to present a useful and truthful picture, an accurate map must tell white lies.” So how can we sort the little white lies from the more serious ones? Where is a map most accurate; where is it least? If it is not correct, what is the next most likely condition? In short, what does it take to get an honest map?
First we need to recognize that these white lies cause maps to 1) distort the 3-D world into a 2-D abstraction, 2) selectively characterize just a few elements from the actual complexity of spatial reality, and 3) attempt to portray environmental gradients and conceptual abstractions as distinct spatial objects.
The first two concerns have challenged geographers and cartographers since the inception of map-making and have resulted in at least de facto standards for most of these issues. The third concern, however, is more recent and involves fuzzy logic and spatial statistics in its expression.
One approach responding to the “fuzzy” nature of maps develops a shadow map of certainty assessing map accuracy throughout its spatial extent. Figure 1 illustrates such an approach using traditional soil and forest maps (left side) and their corresponding maps of certainty (right side). The shaded features on the base maps indicate soil type 2 and forest type 5 (red and green respectively). Traditional mapping assumes that soil type 2 occurs as one consistent glob in the right portion of the study area and forest type 5 occurs as three little globs precisely delineated as shown—a couple white lies.
Just ask the soil scientist or forester who drew the maps. The boundaries are likely somewhere near their delineations, but not necessarily right on. It’s their best guesses, not the latitudes and departures taken with a surveyor’s transit. The challenge for GIS is to differentiate that type of map data (interpreted) from precise map renderings (measured), such as surveyed property boundaries. Furthermore, a geographically distributed assessment of certainty should accompany each map—sort of “glued” to the bottom of the interpreted map’s features.
Figure 1. Maps of soil and forest cover (left) can be linked to their relative certainty (right). Darker tones indicate less certain areas around spatial transitions.
On the right side of the figure, the lines indicate the implied feature boundaries, and the shaded gradient depicts certainty as a function of proximity to an implied boundary. The dark grey areas at the boundaries are assigned a relatively low probability (.5) of correct classification, whereas the interior yellow areas are assigned the highest value (1.0). The shades in between represent increasing likelihood (light grey= .7 and bright blue= .9). This approach reflects a reasonable “first-order” assumption that areas around soil and forest transitions are the least certain, while feature interiors are the most certain.
In a raster system, the shadow map is generated by “spreading” the boundary locations to a specified distance. The resulting proximity map is “renumbered” to indicate probability of correct classification—from .5 for boundary locations to 1.0 for interior locations relatively far away.
In this instance, a linear function of increasing probability was used consistently throughout both maps. However, if warranted, a separate distance function could be developed for each type of boundary transition (e.g., if soils 2 and 3 are easy to delineate, uncertainty might extend only half as far from there implied boundary). Or a non-linear function, such as inverse-distance-squared, could be used. Also, if a feature is frequently “pocked” with other types, the interior certainty value might not attain 1.0 probability.
Admittedly, all this sounds a bit farfetched and a great deal of work for both man and machine, but it does illustrate GIS’s capacity to map a continuum of certainty, as well as simply feature location—an initial step toward an honest map. Subsequent sections will look at other means for certainty mapping and its use in error propagation modeling.
Reference: Monmonier, Mark. 1991. How to Lie with Maps. University of Chicago Press, Chicago, Illinois, USA.
Author's Note: For an overview of map certainty and error propagation issues, see Topic 6, “Overlaying Maps and Characterizing Error Propagation,” Beyond Mapping: Concepts, Algorithms and Issues in GIS (Berry, 1993; GIS World Books) or “Beyond Mapping” columns November-December, 1991, GIS World. Those with tMAP software should review TMAP6.CMD tutorial.
(GIS World, August 1994)
The previous section developed the concept of a shadow map of certainty to express the “fuzzy” nature of some map boundaries. Now let’s extend that concept to error propagation when combining maps. This strikes at the bread-and-butter of a GIS—identifying areas of map coincidence.
The simple intersection of lines on two maps isn’t sufficient, however, for overlaying uncertain maps. Keck, if you weren’t sure about either map’s boundary placement, why would you be certain about their composite pile of spaghetti? What is needed is a composite shadow map of certainty that tells the user where it is likely less valid—an honest assessment of error.
Figure 1 shows the joint coincidence (overlay) of the soil and forest maps described in the previous section. The maps on the left identify the “son and daughter polygons” spawned in the conventional overlay process. The process involves the mathematical intersection of the locational tables associated with the two maps. The upper-left map graphically portrays the results, but keep in mind that the GIS just “sees” a massive table of X, Y coordinates. These coordinates are grouped into the new polygons, and the attribute tables for the soil and forest maps are merged. The result is a link between each new polygon and its joint soil/forest attribute.
The bottom-left map shows the result (yellow polygons) of a geographic search for all of the locations that are soil type 2 and forest type 5. To achieve this feat, the computer merely searched the composite attribute table for the desired joint condition, then plotted the coordinates of the S2F5 polygons to the screen and filled them with a vibrant color. There, that’s it—quick, clean and right-on. More importantly, it’s comfortable. It’s the same thing you would do if you were armed with transparent sheets, pens, light table and vast amounts of patience.
Figure 1. The coincidence of the soil and forest maps can be linked to their joint certainty (insets (c) and d)). Darker tones (purple and dark blue) indicate less certain areas around the simple coincidence of the polygon boundaries.
But how good is the result, considering that uncertainty exists in both of the input maps? Can the GIS account for the propagated error based on the certainty maps?
A first-order approximation of the propagated error involves computing the joint probability by simply multiplying the soil-certainty map times the forest-certainty “shadow” map. The upper-right map in figure 1 shows the resultant distribution of certainty. The dark blue tone is the least certain (.5 x .5= .25) and represents areas of boundary coincidence—really not-to-sure both conditions are there. The yellow tone identifies areas of relative certainty (1.0 x 1.0= 1.0).
The bottom-right map (inset d) isolates the certainty for the geographic search of soil 2 and forest 5. Note that most of the uncertainty is concentrated in the left portion of both resultant polygons. Figure 2 contains a table summarizing these data. About half of the map (50.95 percent) is fairly uncertain of the joint condition (S2F5 probability of <.7). But the results of the traditional overlay procedure implied it’s 100 percent certain that S2F5 occurs throughout the entire resultant polygons—a uniform spatial distribution of “no error anywhere.” Honestly, can you believe that?
Figure 2. A table summarizes S2F5 coincidence certainty. Note that a little over half of the coincidence map (50.95 percent) is fairly uncertain of the joint condition (S2F5 probability <.7) and only nine cells (3.42%) are identified as fairly certain of the joint condition (>.9).
Author's Note: For additional reading, see “Data Models and data Quality: problems and Prospects,” M.F. Goodchild, chapter 8, Environmental Modeling with GIS, 1993, Oxford press, oxford, England and “Uncertainty Issues in GIS,” session theme (three papers), GIS ’94 Proceedings, Polaris Conferences, Vancouver, Canada. For a PowerPoint slide set describing error propagation modeling see http://www.innovativegis.com/basis/BeyondMapping_II/Topic4/Honest.ppt
(GIS World, September 1994)
“Dis information ain’t right …blow ‘em away Bugsy.” Sounds like a line from an old gangster movie. But it is more subversive than that. Dis-information is inaccurate data that appear genuine and are used as if they are accurate. In some respects, that describes a large portion of spatial data populating our GIS databases. The previous two sections developed the concept of a shadow map of certainty and its use in error propagation modeling. The discussion focused on discrete map types (soil and forest) and the probability of correct boundary line placement. Now let’s turn our attention to continuous surface data and related techniques for assessing error.
Continuous mapped data, such as elevation or barometric pressure, are characterized best as surfaces (versus the traditional mapping features of points, lines and polygons). These surfaces are often are derived through spatial interpolation. If you are not too “technologically-numbed,” review the January through March 1994 GeoWorld columns (Beyond Mapping II, Topic 2) discussing spatial interpolation. They establish how the estimates of a mapped variable are derived from field data. But what about the corresponding shadow map of certainty? Where are the estimates good predictors? Where are they bad?
Figure 1 identifies the important considerations in developing a map of certainty, based on the same field data used in the earlier interpolation sections. First, note the circle encompassing the 16 field samples of animal activity. All interpolation procedures establish some sort of “roving window” to identify the measurements to be used in the computations. The windows need not be circular, but can take a variety of shapes and even change shape as they move. In this example, the window is large enough and positioned to capture all the field data when it is operating in the center of the project area. Locations toward the edges of the map must work with less data (only a portion of the full circle).
That is the first order consideration in assessing certainty—the number of sample values used. It is a bit more complex, but it is common sense that if there is only one field measurement in the window the estimate might be less reliable than if there are several. Another consideration is the positioning of the measurements. If the values are at the window extremities, the estimate might be less reliable than if they were close to the center (location to be interpolated). The “weighted nearest-neighbor” algorithm considers the relative distances o the measurements, with the average their distances providing some insight into an estimate’s certainty.
In addition to window shape and data positioning, the data values themselves can contribute to certainty assessment. The left side of figure 1 depicts an implied appraisal based on the data’s Coefficient of Variation (Coffvar), a statistic that tracks the relative variation in the data used in the interpolation. It computes both the average (typical response) and the standard deviation (variability in responses), then computes the percentage of the variability surrounding the average. Interpolated locations based on variable data within the window are assumed less reliable than locations with values that are about the same unless there is a strong directional trend in the data.
The figure identifies four equal steps in variability from 0 percent though 200 percent. The highest Coffvar is 174 percent (viz., not too sure) at the top left, while the lowest is 36 percent in the top right (viz., more sure). The map was “draped” on the interpolation surface of animal activity so the viewer can see the estimated number of animals as the height of the surface combined with color classes of map certainty. It is interesting that the greatest variability (darker red tones) corresponds to the areas of lower animal activity. Any ideas why?
True, the window in these areas captures the 0’s in the northwest corner as well as some of the high values to the east. Foremost in your reasoning, however, should be the simplicity of this technique. For starters, a “weighted” Coffvar considering value positioning might help, or composite statistic considering the size of the window, number of values, their positioning and their variation. How about the alignment with the trend in the data? How might sampling method affect results? What about measurement error versus procedural error? Whew!
Figure 1. Statistics, such as Coefficient of Variation (COFFVAR), can be used to assess spatial interpolation conditions and generate a map of certainty. The lighter tones indicate areas of less certainty because the COFFVAR of the nearby samples is relatively large.
In reality, certainty assessment is a complex area in which spatial statistics has only scrathed the surface. Procedures like Kriging generate a set of shadowed errors each time a set of field measurements is interpolated. Most programs simply discard this valuable information, as the focus is on the estimated surface itself. “Heck, what would users do with a map of error anyway? Let’s give them another 256 colors instead.”
The important point for “mere-mapping-mortals” is not an in-depth understanding of statistical theory, but the recognition that maps contain uncertainty and that procedures are being developed to characterize error distribution and propagation. Yep, GIS has launched us beyond mapping as many of us remember it. Strap yourself in, because it is bound to be an exciting, bumpy ride.
Author's Note: As always, allow me to apologize in advance for the “poetic license” invoked in this terse treatment of a complex subject. Those with access to MapCalc software, enter the command “Scan Data within 13 Coffvar for Coffvar13” to generate figure 1.
Empirical Verification Assesses Mapping Performance
(GIS World, October 1994)
If you want to turn a GIS specialist ashen, suggest taking a map to the field for a little "empirical verification." You know, go to a location and look around to see if the map is correct. If you do that systematically at a lot of locations (keeping track of the number of correct classifications and the nature of the incorrect classifications), you'll learn a lot about the map's certainty. The previous three sections discussed ways you could get the computer to guess about certainty— but empirical verification uses "ground truth" to directly assess mapping performance.
Consider a typical GIS map, such as soil or forest type. What do you know about its certainty? Usually nothing if you're a typical user and you simply clicked on a map in a scroll list. It pops up with finely etched features filled with vibrant colors. But try looking beyond the image to its real-world accuracy. Figure 1 identifies a couple of ways to do this based on a\error matrix summarizing the correct and incorrect cells for a set of test plots.
Figure 1. An Error Matrix reports the proportion of correct classifications along the diagonal and the nature of the incorrect classifications in the off-diagonal elements.
Suppose you had a forest map with discrete classification categories Pine, Oak, and Fir. After swatting mosquitoes and cursing the heat for a couple weeks, you assemble the field verification data into the matrix shown. In the first cell of the table, record the number of times it was mapped as pine when you actually stood in pines (PINE-PINE, correct). Record the errors for pines in the next two cells of the column— the number of times the map said you were in pines, but actually you stood in oaks or firs (PINE-OAK and PINE-FIR, incorrect). The correct and incorrect results for the oak and Fir columns are recorded similarly. Now normalize all of the data in the matrix to the total number of test plots for each category so it is expressed in percent.
Note that the proportion of correct classifications are along the diagonal, and the nature of the incorrect classifications are in the off-diagonal elements— an effective summary of overall mapping performance. The off-diagonal information on errors is particularly useful in identifying overall classification confusion.
But what about the distribution of the errors throughout the project area? A first-order guess involves moving a summary window around a map of the results of the test plots. If you find that the preponderance of the mistakes was in the northwest, you might consider additional field checking in that area. Or, possibly that area was mapped by an individual needing a refresher course (or a pink slip). A useful modification to the procedure is to note just the errors in the roving window that involve the category at the window's focus. That gives you an idea of how well that classification is doing in its general vicinity. It may be that pines frequently are misclassified in the northeast, but exhibit good classification in the southwest.
You also could have the window keep track of the nature of misclassifications— the pines are confused with firs in the northeast. Because there are so many fir stands in that area it makes sense that there are a lot of errors. All that might sound a bit strange, but remember. We’re after an honest map that shows us more than just its best guesses without hinting to their accuracy.
Figure 2. Residual Analysis investigates the difference between a predicted value and the actual value for a set of test locations.
Figure 2 describes a related procedure for verifying continuous data (map surfaces). Residual analysis investigates the difference between a predicted value and the actual value for a set of test locations. Recall the example of spatial interpolation of animal activity nearly beaten to death in the previous discussions. Suppose we cheated the computer and held back some of the field measurements from the interpolation. That would give us a good opportunity to verify its performance against known levels of animal activity (ground truth).
The figure identifies thirteen ground truth locations of measured animal activity. The difference between the predicted and actual values at a location identifies the residual. In the figure, the middle sample plot at X= 23 and Y= 13 is identified in the 3-D map. Its residual is computed as -15 (i.e., 50-65= -15). The sign of a residual indicates whether the estimate was too low (-) or too high (+). The magnitude of the residual indicates how far off the guess was. The percent residual merely normalizes the magnitude of error to the actual value; the higher the percentage, the worse the performance.
To generate the %RESIDUAL (bottom left) the %RESID values in the table were, in turn, interpolated for a map of the percent residual. Note that the northeast and southwest portion of the project area seemed to be on target (+10 percent error), whereas the southeast and northwest portions appear less accurate. The extreme northwest portion is way off (>51 percent error).
I wonder why? Like the discussion in the previous section, the answer probably lies in the assumptions and simplicity of the analysis procedure, as much as it lies in the data themselves. Spatial statistics is a developing field for which theory and practical foundation has yet to be set in concrete. At the moment we may have the cart (GIS) in front of the horse (science), but the idea of an honest map boldly displaying its errors will become a reality in the not too distant future...mark my words.
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