Beyond Mapping I

Topic 6Overlaying Maps and Characterizing Error Propagation


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Beyond Mapping book



GIS Facilitates Error Assessment  discusses potential sources of error when overlaying maps and how “shadow maps” of error and “fuzzy theory” can shed light on the problem

Analyzing the Non-Analytical  describes how “joint probability of coincidence” and “minimum mapping resolution” can be used to assess results of overlaying maps


Note: The processing and figures discussed in this topic were derived using MapCalcTM software.  See to download a free MapCalc Learner version with tutorial materials for classroom and self-learning map analysis concepts and procedures. 


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GIS Facilitates Error Assessment

(GIS World, November 1991)

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…cross an Elephant with a Rhino and what do you get? …El-if-I-Know


Overlay a soils map with a forest cover map, and what do you get?— El-if-I-Know.  It is supposed to be a map that indicates the soil/forest conditions throughout a project area.  But are you sure that's what it is?  Where are you sure the reported coincidence is right-on?  Where are you less sure about the results?  Stated in the traditional opaque academic way... "what is the spatial distribution of probable error associated with your GIS modeling product?" 


It's not enough to simply jam a few maps together and presume the results are inviolately accurate.  You have surely heard of the old adage, "garbage in, garbage out."  But even if you have good data as input, the 'thruput' can garble your output.  Whoa!  That's nonsense.  As long as one purchases a state-of-the-art system and is careful in constructing the database, everything will be OK.  Right?  Well, maybe there is just couple of other things to consider, like map uncertainty and error assessment.


There are two broad types of error in GIS-- those that are present in the encoded base maps, and those that arise during analysis.  The source documents you encode may be inherently wrong.  Or your encoding process may introduce error.  The result in either case, is garbage poised to be converted into more garbage... at megahertz speed and in vibrant colors.  But things aren't that simple, just good or bad data, right or wrong.  The ability of the interpretation process to characterize a map feature comes into play.  


Things always seem a bit more complicated in a GIS.  Traditional 'scaler' mathematical models reside in numeric space, not the seemingly chaotic reality of geographic space.  Spatial data has both a 'what' (thematic attribute) and a 'where' (locational attribute).  That's two avenues to error.  Consider a class of aspiring photo interpretation students.  Some of the students will outline a tightly clustered stand of trees and mark it as ponderosa pine.  Others will extend their boundaries out to encompass a few nearby trees scattered in the adjoining meadow, and similarly mark it as ponderosa pine.  The remainder of the class will trace a different set of boundaries, and mark their renderings as lodgepole pine.  Who's feature definition are you going to treat as gospel in your GIS?


Unless a feature exhibits a sharp edge and is properly surveyed, there is a strong chance that the boundary line is misplaced in at least a few locations.  Even exhibiting a sharp edge may not be enough.  Consider a lake on an aerial photo.  Using your finest tipped pen and drafting skills, your line is bound to be wrong.  A month later the shoreline might recede a hundred yards.  Next spring it could be a hundred yards in the other direction.  So where is the lake?-- El-if-I-Know.  But I do know it is somewhere between here (high water mark) and there (low water mark).  If it's late spring, its most likely near here.


This puts map error in a new light.  Instead of a sharp boundary implying absolute truth, a probability surface can be used.  Consider a typical soil map.  The polygonal edge implies that soil A stops right here, and soil B begins immediately.  Like the boundary between you and your neighbor, the transition space is zero and the characteristics of the adjoining features are absolutely known.  That's not likely the case for soils.  It's more like a probability distribution, but expressed in map space. 

Imagine a typical soil map with a probability 'shadow' map clinging to it-- sort of glued to the bottom.  Your eye goes to a location on the map and notes the most likely soil from the top map, then peers through to the bottom to see how likely it actually is.  For human viewing you could assign a color to each soil type, just like we do now.  Then you could control the 'brightness' of the color based on the soils likelihood-- a washed-out pink if you're not too sure it's soil A; a deep red if you're certain.  That's an interesting map with colors telling you the type of soil (just like before), yet the brightness adds information about map uncertainty.


The computer treats map uncertainty in a similar fashion.  It can store two separate maps (or fields in the attribute table).  One for classification type and another for uncertainty.  Or for efficiency sake, it can use a 'compound' number with the first two digits containing the classification type and the second two digits its likelihood.  Sort of a number sandwich, that can be peeled apart into the two components identifying the 'what' characteristic of maps-- what do you think is there, and how sure are you.


An intriguing concept, but is it practical?  We have enough trouble just preparing traditional maps, little lone a shadow map of probable error.  For starters, how do we currently report map error?  If it is reported at all, it is broadly discussed in the map's legend or appended notes.  For some maps, they are field evaluated and an error matrix is reported.  This involves locating yourself in a project area, noting what is around you, and comparing this with what the map predicts.  Do this a few hundred times and you get a good idea of how well the map is performing-- e.g., ponderosa pine was correctly classified 80% of the time.  If you keep track of the errors that occurred, you also know something about which map features are being confused with others-- e.g., 10% of the time Lodgepole pine was incorrectly identified as ponderosa pine.


Good information.  It alerts us to the reality of map errors and even describes the confusion among map features.  But still, it's a spatially aggregated assessment of error, not a continuous shadow map of error.  However, it does provide insight into one of the elements necessary to the development of the shadow map.  Recall the example of the photo interpretation students.  There are two ways they can go wrong.  They can imprecisely outline the boundary and/or they can inaccurately classify the feature.  The error matrix summarizes classification accuracy.


But what about the precision of a boundary's placement?  That's the realm of fuzzy theory, a new field in mathematics.  You might be sure the boundary is around here somewhere, but not sure of it's exact placement.  If this is the case, you have a transition gradient, not a sharp line.  Imagine a digitized line, separating soil A from soil B.  Now imagine a series of distance zones about the line.  Right at the boundary line, things are pretty unsure, say 50/50 as to whether it is soil A or B.  But as you move away from the implied boundary line, and into the area of soil A, you become more certain it's A.  Each of the distance zones can be assigned a slightly higher confidence, say 60% for the first zone, 70% for the second and so on. 


Keep in mind that the transition may not be a simple linear one, nor the same for all soils.  And recall, the information in the error matrix likely indicates that we never reach 100% certainty.  These factors form the ingredients of the shadow map of error.  Also, they form a challenge to GIS'ers... to conceptualize many map themes in a new, and potentially more realistic way.  GIS is not just the cartographic translation of existing maps into the computer.  The digital map is inherently different from a paper map.  The ability to map uncertainty is a big part of this difference.  The ability to assess error propagation as we analyze these data is another part... reserved for next issue.


As with all Beyond Mapping articles, allow me to apologize in advance for the "poetic license" invoked in this terse treatment of a technical subject.  Two references on GIS error assessment are "Recognition and Assessment of Error in Geographic Information Systems," by S.J. Walsh, et. al., Photogrammetric Engineering and Remote Sensing, October, 1987, Vol. 53(10):1423-1430 and "Accumulation of Thematic Map Error in Digital Overlay Analysis," by J.A. Szajgrin, The American Cartographer, 1984, Vol. 11(1):58-62.



Analyzing the Non-Analytical 

(GIS World, December 1991)

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The previous section took the position that most maps contain error, and some maps actually are riddled with it.  For most readers, this wasn't so much a revelation, as recognition of reality.  Be realistic... a soil or vegetation map is just an estimate of the actual landscape pattern of these features.  No one used a transit to survey the boundaries.  Heck, we're not even positive the classification is right.  Field checking every square inch is out of the question. 


Under some conditions our locational and thematic guesses are pretty good; under other conditions, they're pretty bad.  In light of this, last issue suggested that all maps should contain 'truth in labeling' and direct the user's attention to both the nature and extent of map uncertainty.  You were asked to imagine a typical soil map with a 'shadow' map of probable error glued to its bottom.  That way you could look at the top to see the soil classification, then peer through to the bottom to see how likely that classification is at that location.  Use of an 'error matrix' and 'fuzzy theory' were discussed as procedures for getting a handle on this added information.  More advanced approaches, such as Bayesian, Kriging and other statistical techniques will be discussed in a latter issue.


But, regardless how we derive the 'shadow' map of error, how would we use such information?  Is it worth all the confusion and effort?  In manual map analysis, the interaction of map error is difficult to track and most often deemed not worth the hassle.  However, digital map analysis is an entirely new ball game.  When error propagation comes to play, it takes us beyond mapping and even beyond contemporary GISing.  It extends the concept of Cartographic Modeling to one of Spatial Modeling.  Applications can move from planning models to process models.  Whoa!  This is beginning to sound like an academic beating-- a hundred lashes with arcane terminology longer than a bull whip. 


Actually, it identifies a new frontier for GIS technology-- error propagation modeling.  Assume you want to overlay vegetation and soil maps to identify areas of Douglas fir and Cohasset soil.  But be realistic, you're not completely comfortable with either map's accuracy.  Even if the classifications are correct, the boundaries may not be exactly in the right place.  The simplest error propagation model is the joint probability.  Say two features are barely overlapping as depicted in the left portion of figure 1.  That says, if it's only a coin-flip that it's Douglas fir (0.5), and it's only a coin-flip that it's Cohasset soil (0.5), then it's quite a long shot (0.5 * 0.5= 0.25) that both are present at that location.  The Douglas fir and Cohasset combination is our best estimate, but don't put too much stock in it.  Another map location that is well within the bounds of both features is a lot more certain of the coincidence.  Common sense (and the joint probability of 1.0 * 1.0= 1.0), tells us that. 



Figure 1.  Characterizing map certainty involves 1) error propagation modeling and 2) mixing informational scales (minimum mapping resolution).


An even better estimate of propagated error is a weighted joint probability.  It's a nomenclature mouthful, but an easy concept.  If you are not too sure about a location's classification, but it doesn't have much impact on your model, then don't sweat it.  However, if your model is very sensitive to a map variable, and you're not too sure of its classification, then don't put much stock in the prediction. 


The techy types should immediately recognize that the error propagation weights are the same as the model weights-- e.g., the X coefficients in a regression equation.  In implementation, you spatially evaluate your equation using the maps as variables.  At each map location a set of values are 'pulled' from the stack of maps, the equation solved, and the result assigned to that location on the solution map.  However, you are not done until you 'pull' error estimates from the set 'shadow' maps, compute a weighted joint probability and glue it to the bottom of the solution map.  Vendors and users willing, this capacity will be part of the next generation of GIS packages.


So where does this leave us?  We have, for the most part, translated the basic concepts and procedures of manual map analysis into our GIS packages.  Such concepts as map scale and projection are efficiently accounted for and recorded.  Some packages even pop-up a warning if you try to overlay two maps having different scales.  You can opt to rescale to a common base before proceeding.  But should you?  Is a simple geographic scale adjustment sufficient?  Is there more to this than meets the eye?  The procedure of rescaling is mathematically exacting-- simply multiply the X,Y coordinates by conversion factor.  But the procedure ignores the informational scale implications (viz. thematic error assessment). 


A simple classroom exercise illustrates this concern.  Students use a jeweler's loupe to measure the thickness of a stream depicted on both 1:24,000 and 1:100,000 map sheets.  They are about the same thickness which implies a stream several feet wide at the larger scale, and one tens of feet wide at the smaller scale.  Which measure is correct?  Or is flooding implied?  Using a photo copier they enlarge the 1:100,000 map to match the other one.  The two maps are woefully dissimilar.  The jigs and jags in one is depicted as a fat smooth line in the other.  Which is correct?  Or is stream rechanneling implied? 


The discrepancy is the result of mixing scales.  The copier adjusted the geographic scale, but ignored the informational scale differences.  In a GIS, rescaling is done in a blink of the eye, but it should be done with proper reverence.  Estimation of the error induced should be incorporated in the rescaled product. 

Minimum mapping resolution (MMR) is another aspect of informational scale.  It reports the level of spatial aggregation.  All maps have some level of spatial aggregation.  And with spatial aggregation, comes variation-- a real slap in the face to map accuracy.  A soil map, for example, is only a true representation at the molecular level.  If you aggregate to dust particles, I bet there are a few stray molecules tossed in.  Even a dirt clod map will likely have a few foreign particles tossed in.  In a similar light, does one tree constitute a timber stand on a vegetation map?  Or does it take two?  What about a clump of twenty pines with one hemlock in the center?  The minimum mapping resolution reports the smallest area that will be circled and called one thing.


So what?  Who cares?  Consider the right side of the accompanying figure.  If we overlay the soil and vegetation maps again, and this time identify locations of Serpentine soil and Hemlocks, an interesting conclusion can be drawn-- there is a lot Hemlocks growing in of Serpentine soils.  That's interesting, because foresters tell us that never happens.  But there it is, bright green globs growing in bright orange blobs. 


What is going on here?  Mixing scales again, that's what.  Any photo interpreter can see individual hemlocks in a sea of deciduous trees in winter imagery.  But you don't circle just one, as you're left with just an ink dot.  So you circle stands of about a quarter acre forming small polygons.  Soil mapping is often a tougher task.  You have to look through the vegetation mask, note subtle changes in topography and mix well with a lot of intuition before circling a soil feature.  As of Serpentine soil features are particularly difficult to detect, a five acre polygon is about as small as you go.  However, you are careful to place a marginal note in the legend of the soil map about frequent alluvial pockets of about a quarter acre in the area. 


That's the reality of the of Serpentine and Hemlock coincidence-- the trees are growing on alluvial pockets smaller than the MMR of the soil map.  But the GIS said it was growing in of Serpentine soil.  That's induced error by mixing informational scales.  So what can we do?  The simplest approach is to 'dissolve' any polygonal prodigy that are ridiculously small into their surroundings.  Another approach 'tags' each coincidence feature that is smaller than the coarsest MMR with an error estimate.  Sort of a warning that you may be wrong.


Whew!  Many GIS'ers are content with just going with the best guess of map coincidence and have no use for mapping error.  A cartographic model that automates manual map analysis procedures is often more than sufficient and worth its weight in gold.  Yet there is a growing interest in spatial models with all the rights, privileges and responsibilities of a true map-ematics.  A large part of this rigor is the extension of mathematical procedures, such as error assessment, to GIS technology.  We are just scratching at the surface of this extension.  In doing so, we have uncovered a closet of old skeletons defining map content, structure and use.  The digital map and new analytic capabilities are challenging these historical concepts and rapidly redefining the cartographic playing field.


As with all Beyond Mapping articles, allow me to apologize in advance for the "poetic license" invoked in this terse treatment of a technical subject.  Readers interested in further references should contact NCGIA about their forthcoming Proceedings of the First International Conference on Integrating Geographic Information Systems and Environmental Modeling, held in Boulder, Colorado, September 15-19, 1991 (phone, 805-893-8224).



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