A computer-implemented method of identifying one or more locations along a route for an existing underground duct which indicate a blockage risk, the route extending between two ends, the method comprising: obtaining a terrain elevation profile for the route; estimating a duct elevation profile for the route based on the terrain elevation profile; and predicting one or more blockage risk locations along the route by determining where water entering the duct from each of the two ends would settle, based on the duct elevation profile, by: splitting the duct elevation profile into a plurality of duct portions, wherein for each duct portion the duct elevation profile's gradient over the entire duct portion is one of positive, negative or flat; and predicting a blockage risk location in a first duct portion when at least one of the following conditions i) to iii) is determined to be true for that first duct portion: (i) the first duct portion (221, 224) extends between one of the two ends of the route and a second duct portion, the duct elevation profile being lower over at least a part of the first duct portion than over at least a part of the second duct portion, (ii) the first duct portion (225, 226) is a side of a two-sided well consisting of the first duct portion and a second duct portion (226, 225) extending from a lowest point of the first duct portion and having a gradient of opposite sign to the first duct portion, and (iii) the first duct portion (227, 228, 229) is a side of a three-sided well consisting of a flat duct portion extending between lower ends of a duct portion having a negative gradient and a duct portion having a positive gradient; the method further comprising: generating an overlay for a map of the route, the overlay comprising an indication of each blockage risk location, then causing a display device to display the overlay on the map of the route; and/or in response to predicting the one or more blockage risk locations, causing a user output device to raise a user alert. obtaining a terrain elevation profile for the route; estimating a duct elevation profile for the route based on the terrain elevation profile; and predicting one or more blockage risk locations along the route by determining where water entering the duct from each of the two ends would settle, based on the duct elevation profile, by: splitting the duct elevation profile into a plurality of duct portions, wherein for each duct portion the duct elevation profile's gradient over the entire duct portion is one of positive, negative or flat; and predicting a blockage risk location in a first duct portion when at least one of the following conditions i) to iii) is determined to be true for that first duct portion: (i) the first duct portion (221, 224) extends between one of the two ends of the route and a second duct portion, the duct elevation profile being lower over at least a part of the first duct portion than over at least a part of the second duct portion, (ii) the first duct portion (225, 226) is a side of a two-sided well consisting of the first duct portion and a second duct portion (226, 225) extending from a lowest point of the first duct portion and having a gradient of opposite sign to the first duct portion, and (iii) the first duct portion (227, 228, 229) is a side of a three-sided well consisting of a flat duct portion extending between lower ends of a duct portion having a negative gradient and a duct portion having a positive gradient; splitting the duct elevation profile into a plurality of duct portions, wherein for each duct portion the duct elevation profile's gradient over the entire duct portion is one of positive, negative or flat; and predicting a blockage risk location in a first duct portion when at least one of the following conditions i) to iii) is determined to be true for that first duct portion: (i) the first duct portion (221, 224) extends between one of the two ends of the route and a second duct portion, the duct elevation profile being lower over at least a part of the first duct portion than over at least a part of the second duct portion, (ii) the first duct portion (225, 226) is a side of a two-sided well consisting of the first duct portion and a second duct portion (226, 225) extending from a lowest point of the first duct portion and having a gradient of opposite sign to the first duct portion, and (iii) the first duct portion (227, 228, 229) is a side of a three-sided well consisting of a flat duct portion extending between lower ends of a duct portion having a negative gradient and a duct portion having a positive gradient; (i) the first duct portion (221, 224) extends between one of the two ends of the route and a second duct portion, the duct elevation profile being lower over at least a part of the first duct portion than over at least a part of the second duct portion, (ii) the first duct portion (225, 226) is a side of a two-sided well consisting of the first duct portion and a second duct portion (226, 225) extending from a lowest point of the first duct portion and having a gradient of opposite sign to the first duct portion, and (iii) the first duct portion (227, 228, 229) is a side of a three-sided well consisting of a flat duct portion extending between lower ends of a duct portion having a negative gradient and a duct portion having a positive gradient; generating an overlay for a map of the route, the overlay comprising an indication of each blockage risk location, then causing a display device to display the overlay on the map of the route; and/or in response to predicting the one or more blockage risk locations, causing a user output device to raise a user alert.