ASSESSING the ACCURACY of REMOTELY SENSED DATA - CHAPTER 6

CHAPTER 6 Analysis of Differences in the Error Matrix After testing the error matrix for statistical significance, the next step in analysis involves discovering why some of the accuracy map site labels do not match the reference labels. While much attention is placed on overall accuracy percentages, by far the more interesting analysis concerns learning why sites do not fall on the diagonal of the error matrix. To both effectively use the map and to make better maps in the future, we need to know what causes the differences in the matrix. All differences will be the result of one of four possible sources: 1. Errors in the reference data; 2. Sensitivity of the classification scheme to observer variability; 3. Inappropriateness of the remote sensing technology for mapping a specific land cover class; and 4. Mapping error. This chapter reviews each one of these sources and discusses the impacts of each one to accuracy assessment results. ERRORS IN THE REFERENCE DATA A major assumption of the error matrix is that the label from the reference information represents the “true” label of the site and that all differences between the remotely sensed map classification and the reference data are due to classification and/or delineation error. Unfortunately, error matrices can be inadequate indicators of map error, because they are often confused by errors in the reference data (Congalton and Green, 1993), a function of • Registration differences between the reference data and the remotely sensed map classification caused by delineation and/or digitizing errors. For example, if GPS is not used in the field during accuracy assessment, it is possible for field ©1999 by CRC Press personnel to collect data in the wrong area. Other registration errors can occur when an accuracy assessment site is incorrectly delineated or digitized, or when an existing map used for reference data is not precisely registered to the map being assessed. • Data entry errors. Data entry errors are common in any database project and can be controlled only through rigorous quality control. Developing digital data entry forms that will only allow a certain set of characters for specific fields can catch errors during data entry. One of the best (yet most expensive) methods for catching data entry errors is to enter all data twice and then compare the two data sets. Differences usually indicate an error. • Classification scheme errors. Every accuracy assessment map and reference site must have a label derived from the classification scheme used to create the map. Classification scheme errors occur when personnel misapply the classification scheme to the map or reference data; a common occurrence with complex classi-fication schemes. If the reference data is in a database, then such errors can be avoided or at least highlighted, by programming the classification scheme rules, and using the program to label accuracy assessment sites. Classification scheme errors also occur when the classification scheme used to label the reference site is different from the one used to create the map—a common occurrence when existing data or maps are used as reference data. • Changes in land cover between the date of the remotely sensed data and the date of the reference data. As the second section of Chapter 4 details, landcover change can have a profound effect on accuracy assessment results. Tidal differences, crop or tree harvesting, urban development, fire, and pests all can cause the landscape to change in the time period between capturing the remotely sensed data and accuracy assessment reference data collection. • Mistakes in labeling reference data. Labeling mistakes usually occur because inexperienced personnel are used to collect reference data. Even with experienced personnel, the more detailed the classification scheme the more likely an error will occur. Some conifer andhardwood species are difficult to distinguish onthe ground, much less from aerial photography. Young crops of broccoli, Brussels sprouts, and cauliflower are easily confused. Thus, accuracy assessment must also be completed on the reference data. If photo interpretation is used to assess a map from satellite imagery, then a sample of the photo-interpreted sites must be visited on the ground. If only field data is used, then some of the sites must be visited twice by two different personnel. Table 6.1 summarizes reference data errors discovered during quality control of a recent assessment. Only six of the differences between the map and reference labels were caused by errors in the map. Over two thirds of the differences (85 sites) were caused by mistakes in the reference data. The most significant error occurred from using different classification schemes (50 sites). In this project, National Wetlands Inventory (NWI) maps were used exclusively to map wetlands, i.e., wetlands were defined in the classification scheme to be those areas identified by NWI data as wetlands. However, when accuracy assessment was done, the reference photo interpreters used a different definition of wetlands. The remaining differences were caused by observer variation, discussed in the next section of this chapter. ©1999 by CRC Press Table 6-1 Analysis of Map and Reference Label Differences SENSITIVITY OF THE CLASSIFICATION SCHEME TO OBSERVER VARIABILITY Classification scheme rules often impose discrete boundaries on continuous conditions in nature such as vegetation cover. In situations where classification scheme breaks represent artificial distinctions along a continuum, observer variabil-ity is often difficult to control and, while unavoidable, can have profound effects on accuracy assessment results (Congalton, 1991; Congalton and Green, 1993). Anal-ysis of the error matrix must include investigations concerning how much of the matrix difference results from observers being unable to precisely distinguish between classes when the accuracy assessment site is on the margin between two or more classes in the classification scheme. Plato’s allegory in the cave is useful for thinking about observer variability. In the allegory, Plato describes prisoners who cannot move: Above and behind them a fire is blazing in the distance, and between the fire and the prisoners there is a … screen which marionette players have in front of them over which they show puppets … [The prisoners] see only their own shadows, or the shadows of one another which the fire throws on the opposite wall of the cave …. To them … the truth would be literally nothing but the shadows of the images. (Plato, The Republic, Book VII, 515-B, from Benjamin Jowett’s translation as published in Vintage Classics, Random House, New York, 1991.) Like Plato’s prisoners in the cave, we all perceive the world within the context of our experience. The difference between reality and perceptions of reality is often as fuzzy as Plato’s shadows. Between ourselves and from day to day, our observations and perceptions vary depending on our training, experience, or mood. The analysis in Table 6.1 shows the impact that variation in interpretation can have on accuracy assessment. In the project, two photo interpreters were asked to label the same accuracy assessment reference sites. Almost 30% of the differences between the map and reference label were caused by variation in interpretation. ©1999 by CRC Press Consider, for example, the assessment of a map of tree crown closure with classification scheme rules defining classes as Unvegetated Sparse Light Medium Heavy 0–10%, 11–30%, 31–50%, 51–70% 71–100%. An accuracy assessment reference site from photo interpretation estimated at 45% tree crown cover could feasibly be considered correct with either a Light or Medium label because photo interpretation can be ±10%. The map user would be more concerned with a difference caused by a map label of Unvegetated versus a reference label of Heavy tree crown cover. Differences on class margins are both inevitable and far less significant to the map user than other types of differences. Classification systems sensitive to estimates of vegetative cover are particularly susceptible to this type of confusion in the error matrix. Appendix 1 of this chapter shows the very complex classification scheme rules for a recently completed mapping project of Wrangell–St. Elias National Park in Alaska. The classification scheme is extremely affected by estimates of percent vegetative cover. Sensitivity analysis on 140 accuracy assessment sites revealed that nearly 33% of the sites received new class labels when estimates of vegetative cover were varied by as little as 5%. Several researchers have noted the impact of the variation in human interpre-tation on map results and accuracy assessment (Gong and Chen, 1992; Lowell, 1992; Congalton and Biging, 1992; Congalton and Green, 1993). Gopal and Woodcock (1994) state, “The problem that makes accuracy assessment difficult is that there is ambiguity regarding the appropriate map label for some locations. The situation of one category being exactly right and all other categories being equally and exactly wrong often does not exist.” Lowell (1992) calls for “a new model of space which shows transition zones for boundaries,and polygon attributes as indefinite.” As Congalton and Biging (1992) conclude in their study of the validation of photo interpreted stand type maps, “The differences in how inter-preters delineated stand boundaries was most surprising. We were expecting some shifts in position, but nothing to the extent that we witnessed. This result again demonstrates just how variable forests are and the subjectiveness of photo inter-pretation.” While it is difficult to control observer variation, it is possible to measure the variation and to use the measurements to compensate for differences between ref-erence and map data that are caused not by map error but by variation in interpre-tation. One option is to measure each reference site precisely to reduce observer variance in reference site labels. This method can be prohibitively expensive, usually requiring extensive field sampling. The second option incorporates fuzzy logic into the reference data to compensate for non-error differences between reference and map data and is discussed in Chapter 7. ©1999 by CRC Press INAPPROPRIATENESS OF THE REMOTE SENSING TECHNOLOGY Early satellite remote sensing projects were primarily concerned with testing the viability of various remote sensing technologies for mapping certain types of land cover. Researchers tested the hypotheses of whether or not a technology could be used to detect land use, crop types, or forest types. Many accuracy assessment techniques were developed primarily to test these hypotheses. Recent accuracy assessment is more focused on learning about the reliability of a map for land management or policy analysis. However, some of the differences in the error matrix will be because the map producer was attempting to use a remote sensing technology that was incapable of distinguishing certain class types. Under-standing what differences are caused by the technology is useful to the map producer when the next map is being made. In the Wrangell–St. Elias example cited above, Landsat TM data was employed as the primary remotely sensed data, with 1:60,000 aerial photography as ancillary data. The classification scheme included distinctions between pure and mixed stands of black and white spruce. Accuracy assessment analysis showed consistent success at differentiating pure stands of black versus white spruce. However, consistently differentiating these species in mixed or occasional hybrid stands was found to be unreliable. This phenomenon is not surprising considering the difficulty often asso-ciated with differentiating these species in mixed and hybrid stands from the ground. In other words, remotely sensed data cannot be used to reliably differentiate these two types of conditions. To make the map more reliable, the map user can collapse the classification system across classes. In this example, the non-pure spruce classes of Closed, Open, and Woodland were collapsed into an Unspecified Interior Spruce class. In the difference matrix, Unspecified Interior Spruce map labels were considered to be mapped correctly if they corresponded to a pure or mixed white spruce or black spruce reference site demonstrating the same density class of Closed, Open, or Woodland. For example, a map label of Open Unspecified Interior Spruce was considered to be correctly mapped if its corresponding reference label for the site was Open Black Spruce, Open White Spruce, or Open Black/White Spruce mix. While less informa-tion is displayed on the map, the remaining information is more reliable. MAPPING ERROR The final cause of differences in error matrices are the result of mapping error. Often these are difficult to distinguish from an inappropriate use of remote sensing technology. Usually, they are errors that are particularly obvious and unacceptable. For example, it is not uncommon for an inexperienced remote sensing professional to produce a map of land cover from satellite data that misclassifies northeast facing forests on steep slopes as water. Because water and shadowed wooded slopes both absorb most energy, this type of error is explainable, but unacceptable and avoidable. Many map users will be appalled at this type of error and are not particularly interested in having the electromagnetic spectrum explained to them. However, ©1999 by CRC Press ... - tailieumienphi.vn